FACTORS RELATED TO SHORT BIRTH INTERVAL IN LOW- AND MIDDLE-INCOME COUNTRIES: A SYSTEMATIC REVIEW ; ; © 2021, CIET/PRAM AND FOMWAN This work is licensed under the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/legalcode), which permits unrestricted use, distribution, and reproduction, provided the original work is properly credited. IDRC Grant: 108551-001-Synergies in video edutainment: Child spacing and regional training for rollout in Bauchi, Nigeria (IMCHA) https://creativecommons.org/licenses/by/4.0/legalcode RESEARCH ARTICLE Open Access Factors associated with short birth interval in low- and middle-income countries: a systematic review Juan Pimentel1,2,3* , Umaira Ansari4, Khalid Omer4, Yagana Gidado5, Muhd Chadi Baba5, Neil Andersson1,4 and Anne Cockcroft1,4 Abstract Background: There is ample evidence of associations between short birth interval and adverse maternal and child health outcomes, including infant and maternal mortality. Short birth interval is more common among women in low- and middle-income countries. Identifying actionable aspects of short birth interval is necessary to address the problem. To our knowledge, this is the first systematic review to systematize evidence on risk factors for short birth interval in low- and middle-income countries. Methods: A systematic mixed studies review searched PubMed, Embase, LILACS, and Popline databases for empirical studies on the topic. We included documents in English, Spanish, French, Italian, and Portuguese, without date restriction. Two independent reviewers screened the articles and extracted the data. We used the Mixed Methods Appraisal Tool to conduct a quality appraisal of the included studies. To accommodate variable definition of factors and outcomes, we present only a narrative synthesis of the findings. Results: Forty-three of an initial 2802 documents met inclusion criteria, 30 of them observational studies and 14 published after 2010. Twenty-one studies came from Africa, 18 from Asia, and four from Latin America. Thirty-two reported quantitative studies (16 studies reported odds ratio or relative risk, 16 studies reported hazard ratio), 10 qualitative studies, and one a mixed-methods study. Studies most commonly explored education and age of the mother, previous pregnancy outcome, breastfeeding, contraception, socioeconomic level, parity, and sex of the preceding child. For most factors, studies reported both positive and negative associations with short birth interval. Shorter breastfeeding and female sex of the previous child were the only factors consistently associated with short birth interval. The quantitative and qualitative studies reported largely non-overlapping results. (Continued on next page) © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. * Correspondence: juan.pimentel@mail.mcgill.ca 1CIET/PRAM, Department of Family Medicine, McGill University, 5858 Chemin de la Côte-des-Neiges 3rd Floor, Suite 300, Montreal, Quebec H3S 1Z1, Canada 2Facultad de Medicina, Universidad de La Sabana, Campus Universitario puente del común, Chía, Colombia, CP 250001 Full list of author information is available at the end of the article Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 https://doi.org/10.1186/s12884-020-2852-z http://crossmark.crossref.org/dialog/?doi=10.1186/s12884-020-2852-z&domain=pdf http://orcid.org/0000-0002-6842-3064 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/publicdomain/zero/1.0/ mailto:juan.pimentel@mail.mcgill.ca (Continued from previous page) Conclusions: Promotion of breastfeeding could help to reduce short birth interval and has many other benefits. Addressing the preference for a male child is complex and a longer-term challenge. Future quantitative research could examine associations between birth interval and factors reported in qualitative studies, use longitudinal and experimental designs, ensure consistency in outcome and exposure definitions, and include Latin American countries. Trial registration: Prospectively registered on PROSPERO (International Prospective Register for Systematic Reviews) under registration number CRD42018117654. Keywords: Birth intervals, Developing countries, Breastfeeding, Pregnancy outcome, Systematic review Background Adequately spaced births allow women to recover from previous pregnancies, but both too short or too long inter- vals lead to adverse maternal, perinatal, neonatal, and child health outcomes [1]. The World Health Organization (WHO) currently recommends an interval between the last live birth and the next pregnancy of at least 24months [2], a birth interval of 33months. A meta-analysis published in 2006 reported that short (< 18months) and long (> 59months) intervals between two consecutive pregnancies were associated with preterm birth, low birth weight, and being small for gestational age [1]. Another meta-analysis found an association between a birth interval of less than 24months and infant mortality [3], and others have reported an association between short birth intervals and schizophrenia in the offspring [4]. Shorter interpregnancy intervals are associated with prema- ture membrane rupture, abruptio placentae and placenta previa, and uterine rupture among women with previous caesarean section [5]. Similarly, very long birth intervals (> 5 years) are associated with adverse maternal outcomes such as pre-eclampsia [6]. A systematic review published in 2012 suggested poten- tial mechanisms for the adverse consequences of short birth interval [7]. These include poor maternal nutritional status and folate depletion, suboptimal lactation for the newborn, cervical insufficiency, infections, sibling compe- tition, incomplete healing of the uterus, and abnormal re- modelling of endometrial blood vessels. Short birth intervals may also limit the opportunities for economic development of women and their families [8]. One estimate suggests that around 2 million of the 11 million deaths per year of children under 5 years old could be prevented by avoiding birth intervals of less than 2 years [9]. International bodies such as the WHO and USAID have called for further research and actions to ad- dress short birth interval [2, 10]. Short birth interval is more common among women in low- and middle-income countries, where an estimated 17% of married women of reproductive age are reported to have unmet needs for family planning [11]. For ex- ample, the 2018 Nigerian Demographic and Health Survey (DHS) reported that 19% of married women have unmet family planning needs [12]. Similarly, the 2013 Nigerian DHS found that 23% of women had a birth interval of less than 24months for their last two births and 62% had an interval of less than 36months [13]. Several systematic reviews have examined the conse- quences of birth interval for maternal and child health outcomes [1, 3, 5]. Although a number of individual stud- ies reported on potentially causal factors associated with short birth interval [8, 14, 15], we are not aware of any systematic review that has examined the determinants of short birth interval. Understanding the actionable factors related to short birth interval is crucial to inform efforts to address the problem, particularly in low- and middle- income countries. We therefore conducted a systematic review to examine the factors associated with short birth interval in low- and middle-income countries. Methods We registered the protocol prospectively on PROSPERO (International Prospective Register for Systematic Re- views) under registration number CRD42018117654 [16]. Our research question was: what are the factors associated with birth interval in low- and middle-income countries? We report our systematic review following the Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA) guidelines [17]. Our PRISMA check- list is available in Additional file 1. Search strategy Using key terms specific to each database, Boolean opera- tors, and truncators, we developed the search strategy with the collaboration of an experienced librarian from McGill University. We included PubMed, Embase, LILACS, and Popline, which is a database specialized in maternal and reproductive health containing grey literature [18]. Our search strategy is available in Additional file 2. Eligibility criteria We used the following inclusion criteria: (i) experimental or observational studies reporting a measure of association (such as relative risk, odds ratio, or hazard ratio), Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 2 of 17 https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=117654&VersionID=1290849 qualitative studies, and mixed methods studies; (ii) studies in English, Spanish, French, Italian, and Portuguese; (iii) studies conducted in low- or middle-income countries ac- cording to the World Bank classification [19]; (iv) expos- ure: including but not limited to community-based factors, demographic factors, epidemiologic factors, socio- economic factors, environmental factors, and sociocultural factors; (v) outcome: length of intervals between births to women in the population in any time unit. We did not have restrictions regarding the year of publication. We excluded theoretical publications, quantitative studies reporting only proportions or p-values (without a measure of magnitude of association), studies from high-income countries, studies using “birth spacing” as a synonym for contraception use, and studies exploring factors associated with the interval between marriage and first birth. Data collection Using the open-source systematic review web applica- tion Rayyan [20], two independent researchers (JP and UA) screened the titles and abstracts and included only the potentially relevant articles. They resolved discrepan- cies by discussion and consensus, involving a third party (AC) in case of no resolution. Subsequently, we retrieved the full-text articles of all the selected references and re- moved the duplicates using EndNote X8.2. JP and UA performed the final selection of studies using an eligibil- ity format based on the elements listed in the search strategy. We piloted this format on 5% of the retrieved studies. The research team collectively designed the data ex- traction form based on the variables that would answer the research question, in an iterative process with regu- lar meetings to discuss and update the form. JP and UA independently piloted the data extraction form on 5% of the studies to determine its appropriateness. They then extracted data from the included studies and conducted a cross-check review to verify the quality and accuracy of the extracted data. We extracted the following data when available: basic study information (title, year of publication, country, au- thors, type of document, and journal); population and set- ting (sample size, age range, ethnicity, study setting); study methods (aim, study design, unit of analysis, statistical methods, qualitative methods used, ethical approval); and results (exposure and outcome definition, crude and ad- justed measure of association, confidence interval, number of participants with/without the outcome in the exposed/ non-exposed groups, qualitative findings). We planned to conduct a quantitative synthesis only if the included studies were sufficiently homogeneous and of adequate quality. As definition of factors was hetero- geneous and study quality was variable, we performed a narrative synthesis of the quantitative findings and an inductive thematic analysis [21] on the results of the qualitative studies. Methodological quality To assess the quality of the included studies, we used the 2018 Mixed Methods Appraisal Tool (MMAT), a quality appraisal instrument for systematic reviews including quali- tative, quantitative, and mixed methods studies [22]. Re- searchers have reported on this tool’s efficiency, reliability [23], and content validity [24]. JP and UA independently performed the quality assessment of each publication. Results From an initial 2802 documents identified by our search, 43 studies remained after screening and assessment (Add- itional file 3). We categorized these into four subgroups: studies reporting odds ratio (OR) or relative risk (RR); studies reporting hazard ratio (HR); qualitative studies; and mixed methods studies (Fig. 1 and Additional file 4). Figure 2 shows the countries where the studies took place. Africa produced the most studies (21/43), followed by Asia (18), and the Americas (four). The countries with the most studies were Nigeria (five studies), Tanzania (five studies), and India (four studies). Some 14 studies were published after 2010, 14 studies between 2001 and 2010, eight studies between 1990 and 2000, and seven studies before 1990. Four documents were reports (two from grey literature) and the remainder were journal articles. Most of the quantitative studies used a cross-sectional design (24/32) and sample sizes ranged widely from 134 to 64, 943 (Table 1). Studies reporting OR/RR Some 11 studies reported factors associated with short birth interval after adjusting for confounders. The most com- monly reported factors were age of the mother (seven stud- ies), education of the mother (six), and contraception use (five). Five studies defined short birth interval as < 24 months, while two studies used < 33months and < 36 months, respectively (two studies did not provide their def- inition of short birth interval). For all factors but length of breastfeeding (three studies) and sex of the previous child (two studies), researchers reported mixed results. Table 2 shows the associations reported between factors and short birth interval. Seven studies found an association between a short birth interval and a younger age of the mother. Among these publications, two studies explored age at last deliv- ery, one study explored the age of the mother at the mo- ment of the study, and the remaining studies did not specify a definition of age of the mother. Two studies re- ported an association between a short birth interval and older age of the mother (one study explored age at first pregnancy). Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 3 of 17 Three studies found an association between short birth interval with shorter duration of breastfeeding. One study found more short birth interval with no breast- feeding as opposed to exclusive or mixed breastfeeding. Three studies reported an association between a short birth interval and no contraception use, while one publi- cation reported the contrary. The latter study used data from Demographic and Health Surveys from nine sub- Saharan African countries between 1991 and 2001. Five studies reported more short birth interval with less education, but one study reported more short birth interval with more education. Two studies reported an association between short birth interval and less parity, while one study reported the contrary. Two studies reported more short birth interval with lower income, while one study reported the contrary. Similarly, a study reported an association between short birth interval and fewer assets. One study reported more short birth interval for people living in rural areas com- pared with people living in urban settings. Three studies reported an association between short birth interval and an adverse outcome of a previous pregnancy, but one study found longer birth interval after a previous abortion. One study found more short Fig. 1 PRISMA flow diagram Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 4 of 17 birth interval after a singleton birth compared with after a multiple birth. One study found more short birth interval when the place of delivery of the index child was not a health facility. Two studies found more short birth interval when the sex of the previous child was female. An experimental study found more short birth interval with maternal and newborn health care only, compared with integrated post-partum family planning and mater- nal and newborn health care. Four studies reported other factors associated with birth intervals such as marital status of the mother, occupation of husband, place of residence, year, and religion (Table 2). After adjusting for confounders, six studies reported non- significant associations between different factors and short birth interval. The studies explored age of the mother at the moment of the study (four publications), education of the mother (three), contraception method, marital status of the mother, occupation of the mother, and place of residence (two publications each category). Other factors examined included contraception use, age at last delivery, age of the woman’s mother at first pregnancy, type of breastfeeding, occupation of the husband, parity of the mother, delivery place of the previous pregnancy, planned previous preg- nancy, and income level (one publication each category). Studies reporting HR Interpretation of time-to-event data is challenging since the nature of association may change over time. For ex- ample, more education can lead to longer birth intervals in the first two births, but it may have the opposite effect for subsequent births. We have summarized the authors’ overall conclusions for each study (Additional file 4). Among the 16 studies reporting HR, researchers most frequently reported on factors such as education (eight studies), previous pregnancy outcome (seven), age of the mother (six), breastfeeding (four), and socioeconomic level (three). All the studies reported mixed results for all factors except for breastfeeding, for which four stud- ies found more short birth interval with shorter breast- feeding in all the pregnancies. Three studies reported shorter birth intervals with a younger age of the mother, while one study reported the contrary. Two studies that looked at age at marriage and birth interval reported mixed results, and one study found shorter birth intervals with younger age at first birth. Four studies found shorter birth intervals with lower education of the mother, while three studies found the contrary, and two other studies found mixed results. Five studies found shorter birth intervals with adverse out- comes in any previous pregnancy, while one study found the opposite. Two studies found mixed results. One study reported a shorter birth interval with round- worm (Ascaris lumbricoides) infection but extended inter- birth intervals with hookworm infection. Another study found longer birth intervals among women with HIV infection. One study reported shorter birth intervals among mar- ried couples, while another study found shorter birth Fig. 2 Countries where the studies were conducted. Created and reproduced with permission from the open-source web application mapchart.net [25]. Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 5 of 17 http://mapchart.net Table 1 Characteristics of included studies First author and year of publication Country Sample size Unit of analysis Age range/mean (years) Aim of the study Studies reporting OR/RR Abdullah 2018a Bangladesh 4504 Women 15 to 35+ “examines the feasibility of integrating a post-partum family planning intervention package within a community-based maternal and newborn health intervention package, and evaluates the impact of integration on reduction of rates of short birth intervals and preterm births.” Hailu 2016b Ethiopia 636 Women Mean 31 (SD ± 5.16) “assess determinants of interbirth interval among child-bearing age women who have at least two consecutive live births in Arba Minch ZuriaWoreda, SNNP, Ethiopia, 2014.” Chirwa 2014c Democratic Republic of Congo 7172 Women 15 to 49 “investigate the proportion of short birth intervals at the provincial level among young women in the DRC.” de Jonge 2014c Bangladesh 5571 Births NR “identify predictors of short birth interval and determine consequences of short intervals on pregnancy outcomes.” Begna 2013b Ethiopia 636 Women 20 to 49 “assess the determinants of inter birth interval among women’s of childbearing age in Yaballo Woreda, Borena zone, Oromia Regional State, Ethiopia.” Dim 2013c Nigeria 420 Women 20 to 44 “determined the duration of inter-birth interval and the determinants of short inter-birth interval in Enugu, Nigeria.” Muganyizi 2013c Tanzania 427 Women 15 to 45mean 29.2(SD ± 5.1) “explored if the use of modern family planning promotes healthy timing and spacing of pregnancy among women seeking antenatal services.” Exavery 2012d Tanzania 8980 Women 16 to 49 “(1) describe the median level of inter-birth interval (in months), (2) estimate proportions of inter-birth intervals below the recommended minimum inter-birth interval by characteristics of mother and child, and (3) identify factors associated with non-adherence to the recommended minimum inter-birth interval among multiparous women of childbearing age in Rufiji district of Tanzania.” Fayehun 2011c Nigeria 22,752 Births 15 to 49 “examine the effects of demographic, socioeconomic and socio-cultural factors on birth spacing among Nigerian ethnic groups.” Ismail 2008c Malaysia 355 Women Mean 33.5 (SD ± 5.0) “determine the prevalence and associated factors for short birth spacing among Malay women who delivered at Hospital Universiti Sains Malaysia, Kota Bharu, Kelantan.” Todd 2008c Afghanistan 4452 Women Mean 25 (SD ± 5.7) “assess prevalence and correlates of prior contraceptive use among hospitalized obstetric patients in Kabul, Afghanistan.” Ngianga-Bakwin 2005c Nine countries in Africae 50,596 Birth intervals NR “investigate associations between use of depot- medroxyprogesterone acetate and other reversible contraception and short birth intervals in sub-Saharan Africa.” Sirivong 2003c Laos 298 Women 15 to 49 “find out whether or not the training of traditional birth attendants had an impact on reproductive health.” Atkin 1992d Mexico 137 Women (adolescents) < 18 “identifies and explores selected background, pregnancy, and postpartum predictors of short-interval repeat pregnancy among urban Mexican adolescents who were single when they conceived their first pregnancy.” Achadi 1991c Indonesia 6826 Birth intervals NR “examine the relative impact of breastfeeding and family planning use on birth spacing patterns in two major regions of Indonesia.” Franca-Junior 1985c Brazil 345 Children NR “investigate the interpregnancy interval and its relationship with breastfeeding”(translated from Portuguese). Studies reporting HR Blackwell 2015d Bolivia 986 Women NR “investigated associations between intestinal helminths and fertility in women.” Mattison 2015c Tanzania 315 Children 2 to 7mean 4.5 (SD ± 1.6) “ask whether breastfeeding for more than 2 years is associated with discernible health and well-being benefits to children.” Erfani 2014c Iran 9071 Women 15 to 49 “study the determinants of change in the timing of births.” Fallahzadeh 2013c Iran 400 Women 15 to 49 “identify the duration and determinants of inter birth intervals among women of reproductive age in the city of Yazd.” Singh 2012c India 7624 Women NR “see the effect of breastfeeding as a time-varying and time- dependent factor on birth spacing in order to provide input to policy planners.” Dommaraju 2008c India 64,943 Women 15 to 49 “investigates the complex relationship between marriage age and marital fertility by examining the initiation of childbearing and the transition to higher order births by marriage cohorts in India.” Hossain 2007d Bangladesh 31,324 Birth intervals Mean 21.8 “examine the relationship between child mortality and subsequent fertility.” Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 6 of 17 Table 1 Characteristics of included studies (Continued) First author and year of publication Country Sample size Unit of analysis Age range/mean (years) Aim of the study Ly 2006ba Senegal 134 Children and their moms 3.5 months old babies “assess the effects of early, short-term food supplementation of infants (from 4 to 7months of age) on maternal weight change, duration of breastfeeding and birth interval in a rural West African community.” Gyimah 2005c Ghana 10,975 Women 15 to 49 “[examines] the relative socio-economic vis-a-vis socio-cultural factors on the timing of births.” Upadhyay 2005c Philippines 1123 Women 26 to 49mean 37 “look at whether women’s status and autonomy affect birth-to-conception intervals.” Youssef 2005c Jordan 4349 Birth intervals 15–49mean 32.2(SD ± 7.1) “identify the duration and determinants of interbirth intervals among women of reproductive age in one region of Jordan.” van Eijk 2004c Kenya 2218 Women 14 to 30+ “studied factors associated with short pregnancy interval (PI) and the effect of PI on birthweight and haemoglobin.” Hoa 1996c Vietnam 1132 Women NR “explore the reproductive pattern of women in rural Vietnam in relation to the existing family planning policies and laws.” Nair 1996c India 1829 Women < 35 “examine changes in the timing of birth and the important factors determining birth intervals.” Adewuyi 1990c Nigeria 8818 Women NR “examine regional variations in birth interval length as reported in the Nigerian Fertility Survey and the pattern in the variation of birth interval length at different parities. [...] examination of the correlates of birth interval length in the country.” Lehrer 1984c Malaysia 1200 Women < 50 “test the hypothesis that the impact of child mortality on spacing varies across parities.” Mixed-methods studies Dehne 2003 Burkina Faso 350 Community members 15–49 “document current trends in knowledge of, attitudes towards, and relating to traditional and modern child-spacing methods in a remote area in northern Burkina Faso.” Qualitative studies De Vera 2007 Philippines 7 Couples (husbands and wives) 20 to 47 “describe perceptions of birth spacing among rural Filipino husbands and wives.” Social & Rural Research Institute 2003 India 34 Focus groups 17 to 30 “understand knowledge, attitudes, behaviors and practices with respect to birth spacing; determine the factors that motivate birth spacing among those who practice spacing; identify barriers to adoption of spacing methods; understanding knowledge, attitudes and practices of health personnel and institutional support towards birth-spacing.” Dean 1994 Kenya 153 participants Community groups NR “examine the beliefs held and concepts behind childbearing practices in the rural communities of West Pokot District in Kenya and the concrete changes in these practices that have occurred.” Chad Ministry of Public Health 1992 Chad 16 focus groups with 160 men and women Focus groups 18 to 40 “1. Learn how Chadian men and women feel about the concept of family wellbeing; 2. Explore men and women’s understanding of modern family planning methods and family well-being, including rumors and misconceptions; 3. Examine the influence of religion on the use of family planning among Chadian men and women; 4. Examine the image Chadian women and men have of a family planning user.” Kiluvia 1991 Tanzania 50 Focus groups 15 to 35+ “identify persuasive, educational, and appealing family planning messages for radio and print materials. […] To learn why Tanzanian couples choose to space their births.” Van de Walle 1986 Burkina Faso 80 Women NR “revisited 80 women, for a longer description of their postpartum experiences.” Millard 1984 Mexico 285 Women > 15 “shows how cultural systems, in addition to biological constraints, shape lactation patterns and endow breastfeeding with social significance.” Lovel 1983 Zimbawe 204 Women NR “women with at least one child under five were asked about reasons for birth spacing in their parents’ generation.” Adeokun 1982 Nigeria NR Families NR “marital sexual relationships (MSR) and the timing of the next child among the Ekiti and Ikale sub-groups of the Yoruba.” Adeokun 1981 Nigeria 24 Families NR “investigate the patterns of maternal and child care, the parents’ perception of the timing of various milestones in the development of their children, and to seek the links between marital sexuality, child development and the timing of a next child (other than the first).” NR not reported aExperimental bCase-control cCross-sectional dCohort/longitudinal eBurkina Faso, Cameroon, Ghana, Kenya, Madagascar, Malawi, Niger, Tanzania, Zambia Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 7 of 17 Table 2 Adjusted OR/RR and confidence interval of factors significantly associated with birth spacing Category / subcategory Author / year of publication Factor Outcome (length of birth interval in months) Adjusted effect size Confidence interval 95% Age At first pregnancy (years) de Jonge 2014 18.3 vs 18.6 < 33 0.95 0.92–0.98 At delivery of the last child (years) Hailu 2016 17–24 vs > 34 < 36 0.98 0.36–2.66 25–29 vs > 34 0.9 0.40–2.0 30–34 vs > 34 2.58 1.08–5.15 de Jonge 2014 22 vs 22.88 < 33 1.11 1.08–1.15 Of the mother (years)a Chirwa 2014 15–19 vs 45–49 < 25 2.51 1.56–4.04 20–24 vs 45–49 1.79 1.27–2.52 25–29 vs 45–49 1.3 0.93–1.84 30–34 vs 45–49 1.25 0.89–1.76 35–39 vs 45–49 1.05 0.74–1.5 40–44 vs 45–49 0.99 0.69–1.44 Of the mother (years)b Begna 2013 20–24 vs 25–29 < 36 1.36 0.53–3.48 30–34 vs 25–29 0.68 0.39–1.17 35–39 vs 25–29 0.31 0.17–0.6 40–44 vs 25–29 0.22 0.10–0.49 45–49 vs 25–29 0.39 0.15–1.01 Muganyizi 2013 30 vs 15–29 < 24 or > 60 1 0.5–1.7 Exavery 2012 15–19 vs 45–49 < 33 13.65 9.63–19.35 20–24 vs 45–49 4.3 3.16–5.86 25–29 vs 45–49 2.4 1.77–3.26 30–34 vs 45–49 2.07 1.52–2.8 35–39 vs 45–49 1.64 1.21–2.24 40–44 vs 45–49 1.31 0.95–1.83 Ismail 2008 One-year increase in age < 24 0.86 0.8–0.92 Ngianga- Bakwin 2005 < 21 vs > 21 and < 35 < 24 0.58 0.55–0.62 > 35 vs > 21 and < 35 0.67 0.62–0.72 Of the woman’s own mother at first pregnancy (years) Atkin 1992 11–17 vs > 17 < 24 5.1 Not provided Breastfeeding Duration (months) Hailu 2016 12–23 vs > 23 < 36 60.19 31.61– 114.59 Begna 2013 < 25 vs > 24 < 36 30.81 6.97– 136.19 Ismail 2008 < 12 vs > 11 < 24 6.18 3.59–10.62 Type Chirwa 2014 Exclusive breastfeeding vs Mixed < 25 1.08 1–1.17 Never breastfeeding vs Mixed 1.07 0.99–1.15 Ngianga- Bakwin 2005 Exclusive breastfeeding vs No breastfeeding < 24 0.67 0.58–0.78 Mixed feeding vs No breastfeeding 0.86 0.82–0.90 Contraception Method Chirwa 2014 Not using contraception vs Modern method < 25 0.97 0.85–1.08 Ngianga- Bakwin 2005 Using injections vs Using other methods < 24 1.23 1.1–1.38 Atkin 1992 Postpartum IUD: No/Yes < 24 26.34 Not provided Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 8 of 17 Table 2 Adjusted OR/RR and confidence interval of factors significantly associated with birth spacing (Continued) Category / subcategory Author / year of publication Factor Outcome (length of birth interval in months) Adjusted effect size Confidence interval 95% Use Hailu 2016 No vs Yes < 36 3.01 1.68–5.39 Muganyizi 2013 No vs Yes < 24 or > 60 1 0.6–1.8 Begna 2013 No vs Yes < 36 5.91 4.02–8.69 Ismail 2008 No vs Yes < 24 3.95 2.21–7.05 Ngianga- Bakwin 2005 No vs Yes < 24 0.88 0.82–0.93 Todd 2008 Prior contraceptive use: Yes/No Longer mean birth interval (2.21 ± 0.79 Vs 2.01 ± 0.87 years) 1.25 1.12–1.40 Education of the mother Hailu 2016 No formal education vs Has formal education < 36 3.4 1.8–6.43 Chirwa 2014 No education vs Secondary or higher < 25 1.08 0.97–1.19 Primary education vs Secondary and higher 1.06 1–1.16 de Jonge 2014 Secondary or above vs None or primary education < 33 1.26 1.09–1.45 Begna 2013 No formal education vs Formal education < 36 1.89 1.15–3.37 Muganyizi 2013 Primary vs No education < 24 or > 60 1 0.2–4.6 Secondary or above vs No education 1.6 0.3–7.3 Exavery 2012 Never been to school vs Secondary/higher < 33 1.27 1.01–1.60 Primary vs Secondary/higher 1.09 0.87–1.37 Ngianga- Bakwin 2005 No education vs Secondary education or higher < 24 1.16 1.06–1.26 Primary education vs Secondary education or higher 1.11 1.03–1.20 Sirivong 2003 Literate vs Illiterate Birth spacing (length not specified) 0.27 0.08–0.84 Marital status Muganyizi 2013 Not in marriage vs In marriage < 24 or > 60 0.9 0.4–2.1 Exavery 2012 Previously married (widowed or divorced) vs Married < 33 0.56 0.48–0.66 Single vs Married 0.64 0.57–0.73 Atkin 1992 Women who were in a legal or consensual union at 5 months: Yes / No < 24 6.9 Not provided Miscellaneous Intervention Abdullah 2018 Intervention: integrated post-partum family planning and mater- nal and newborn health. Control: maternal and newborn health only. < 24 0.81 0.69–0.95 Occupational group de Jonge 2014 Tea garden resident: Yes vs No < 33 1.41 1.07–1.87 Religion Other vs Muslim 0.68 0.53–0.87 Time period Ngianga- Bakwin 2005 1998–2001 vs 1991–1993 < 24 0.9 0.84–0.95 Occupation Husband Begna 2013 Daily worker vs Animal husbandry < 36 2.19 1.01–4.79 Farmers vs Animal husbandry 0.49 0.24–1 Merchant vs Animal husbandry 0.72 0.36–1.43 Others vs Animal husbandry 1.17 0.47–5.92 Mother Hailu 2016 Farmer vs Others < 36 2.68 0.31–23.23 Housewife vs Others 1 0.17–5.86 Merchant vs Others 1.46 0.16–13.24 Muganyizi Employment Business vs Salary employment < 24 or > 60 1.1 0.6–2.1 Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 9 of 17 Table 2 Adjusted OR/RR and confidence interval of factors significantly associated with birth spacing (Continued) Category / subcategory Author / year of publication Factor Outcome (length of birth interval in months) Adjusted effect size Confidence interval 95% 2013 Employment Housewife/others vs Salary employment 1.6 0.7–3.4 Parity (children) de Jonge 2014 > 3 vs 1 < 33 0.28 0.19–0.41 2 vs 1 0.53 0.44–0.63 3 vs 1 0.38 0.29–0.51 Muganyizi 2013 > 3 vs 2 < 24 or > 60 1.8 0.9–3.7 3 vs 2 0.9 0.5–1.5 Begna 2013 2 vs > 4 < 36 3.73 1.50–9.25 3&4 vs > 4 2.69 1.23–5.92 Exavery 2012 > 3 vs 2 < 33 2.54 2.25–2.85 3 vs 2 1.29 1.19–1.40 Ismail 2008 Parity (no further explanation) < 24 1.46 1.22–1.76 Previous pregnancy Outcome de Jonge 2014 Adverse outcome of any previous pregnancy: Yes / No < 33 2.1 1.83–2.40 Muganyizi 2013 Immediate past pregnancy loss: Yes / No < 24 or > 60 2.5 1.3–4.7 Exavery 2012 Birth: Multiple vs Singleton < 33 0.74 0.57–0.96 Ismail 2008 History of abortion: Yes vs No < 24 0.09 0.02–0.34 Place of delivery Hailu 2016 Health institution vs Home < 36 1.53 0.61–3.8 Exavery 2012 Elsewhere vs Health facility < 33 1.85 1.71–2 Planned Hailu 2016 No vs Yes < 36 1.44 0.9–2.61 Sex of the previous child Hailu 2016 Female vs Male < 36 6.79 3.65–12.63 Begna 2013 Female vs Male < 36 1.72 1.17–2.52 Socioeconomic level Household assetsc de Jonge 2014 0–3 vs > 3 < 33 1.42 1.22–1.65 Income Hailu 2016 Wealth index: Fourth vs Richest < 36 3.96 1.41–11.13 Wealth index: Middle vs Richest 3.98 1.39–11.38 Wealth index: Second vs Richest 6.46 2.26–8.48 Wealth index: Poorest vs Richest 14.33 4.65–44.15 Chirwa 2014 Low vs High < 25 0.98 0.8–1.01 Middle vs High 0.86 0.77–0.94 Ngianga- Bakwin 2005 Low vs High < 24 1.18 1.10–1.26 Middle vs High 1.25 1.17–1.34 Place of residence Chirwa 2014 Rural vs Urban < 25 1.07 0.97–1.13 Exavery 2012 Rural vs Urban < 33 1.04 0.95–1.13 Ngianga- Bakwin 2005 Urban vs Rural < 24 0.85 0.79–0.9 Significant results are shown in bold aWhen the study was conducted bDid not specify a definition of age of the mother cElectricity, radio/tape recorder, fan, TV, fridge, phone, generator and bicycle Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 10 of 17 intervals with shorter duration of the marriage. Regard- ing socioeconomic level, a study found shorter birth in- tervals with lower income, another study found a shorter birth interval with rural residence, and a third study found mixed results (place of residence). Six studies found shorter birth intervals with less par- ity, postpartum amenorrhea less than 6 months, when the previous child is a girl, not using modern contracep- tion, when religion is Catholic, farmer as a profession, and less female decision-making autonomy. An interven- tion study found longer birth intervals with early short- term infant supplementation. A study found mixed re- sults on birth interval for media exposure and employ- ment status of the women. Qualitative and mixed-methods studies We identified nine themes among the findings of the qualitative studies. The leading theme was local concepts and practices (14 codes), followed by modernization (eight), communication (six), religion (four), breastfeeding, health concerns, knowledge and attitudes, miscellaneous (two codes each theme), and autonomy (one). According to the participants of the studies, the obser- vance of local traditions prevents short birth interval. Ex- amples include agbon, described as a “female body odor after parturition” that is to be respected before restarting sexual activity; apa, which requires avoiding a conflictive situation between children if spaced too closely; and ratat, a traditional period of abstinence. Researchers also re- ported the influence of local medicine people (hilots, mar- abu), traditional medicine, use of amulets, talismans, and cords, coitus interruptus, polygamy, and social taboo on the length of birth interval. Modernization was another concept reported by re- searchers. The participants of the studies explained that modernization introduces social changes influencing birth intervals such as loss of culture and traditions, men stay- ing at home more often, availability of health services, education, and food, and changes in religion and beliefs. Availability of health services and infrastructure, and fam- ily planning education prolong birth interval, while the rest of the factors related to modernization promote short birth interval. According to the participants of the qualitative studies, birth interval was also affected by communication be- tween couples and families, the influence of local media and the society, and observation of other parents. Con- cerns for maternal and child health were mentioned as tending to increase birth interval, as was knowledge about contraception methods. Some participants mentioned that breastfeeding prolongs birth interval, while other participants questioned its effect- iveness as a means of preventing an early next pregnancy. Finally, Catholicism and Hinduism were considered to shorten birth spacing, and women’s autonomy, drought, and war were also mentioned to affect the timing of births. Table 3 gives a list of themes, codes, and quotations. Meta-analysis We decided not to conduct a meta-analysis. The in- cluded quantitative studies were very heterogeneous in their definitions of exposures and outcomes, and most used a cross-sectional design with variable approaches to dealing with potential confounders. Quality assessment The quality of most documents ranked as medium (23 documents), followed by high (16 documents), and low (four documents). For information about the quality as- sessment please see Additional file 5. Discussion Our systematic review shows two factors consistently as- sociated with short birth interval: shorter breastfeeding and a female previous child. Younger age of the mother, less education of the mother, a negative outcome of the previous pregnancy, and lower socioeconomic status were often associated with short birth interval, although some studies reported the opposite. The quantitative studies examined a limited number of factors that could be easily included in a questionnaire. Quantitative findings A contraceptive effect of breastfeeding has long been recognised to prolong birth interval. Breastfeeding causes lactational amenorrhea because the suckling stimulus downregulates hypothalamic gonadotropin-releasing hor- mone secretion and the production of luteinizing hor- mone [26]. In 1988, an international group of scientists met at Bellagio, Italy [27], with the support of the WHO, the Rockefeller Foundation, and Family Health International. The group discussed the role of breastfeeding in family planning and concluded that lactational amenorrhoea can be used as a method of contraception. Institutions have followed this advice for decades. The 2017 UK Fac- ulty of Sexual & Reproductive Healthcare Guideline on Contraception After Pregnancy [28] stated that “women may be advised that, if they are less than 6 months post- partum, amenorrhoeic and fully breastfeeding, the lacta- tional amenorrhoea method (LAM) is a highly effective method of contraception.”p21. Despite current efforts to promote breastfeeding worldwide, early weaning is still common and few chil- dren receive exclusive breastfeeding by the age of 6 months [29]. A recent review conducted by Bellù [30] found that breastfeeding support is complex and in- volves individual, structural, and environmental factors. Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 11 of 17 Table 3 Factors associated with birth spacing from qualitative studies Author and year of publication Theme Factor Quotation Page Social & Rural Research Institute 2006 Autonomy Lack of decision- making power “Lack of decision-making powers among the women due to the patriarchal structure of the family that gives the man the reins of power.” 4 Millard 1984 Breastfeeding Breastfeeding does not work “When the issue of lactation contraception is raised with village women, they deny its effectiveness, confidently and laughingly pointing to their own repeated experiences of returning to pregnancy while breastfeeding. They volunteered information no about the return of menses in relation to the probability of conception. Of the women queried about lactation contraception, only one woman in Amanalco stated that she thought breastfeeding probably did reduce the chance of a rapidly ensuing pregnancy.” 9 Adeokun 1982 Breastfeeding works “I do not worry my mind about pregnancy, we keep on having sex and as long as she keeps on breast-feeding nothing can happen. Once she has stopped and her period has returned, then we know it is time for her to be- come pregnant.” 10 De Vera 2007 Communication Couple “The subcategory, lack of communication, was identified as one of the reasons that couples did not space births.” 242 Adeokun 1982 Family “The openness of Yoruba families encourages kin and in-laws to influence a couple’s decisions about child rearing and the timing of the next child.” 12 De Vera 2007 “Family and friends influenced couples’ decisions about birth spacing. Some participants received advice on birth spacing from relatives and friends.” 244 Social & Rural Research Institute 2004 “Familial and cultural factors that determine the extent of support lent by the family (or the lack of it) to the woman. Given the nature of the Indian society, the influence of the peers and the community is critical.” 4 De Vera 2007 Media “The popular media (print and broadcast media) may exert influence on couples’ decisions about spacing births in some communities.” 244 Observation “Whereas others observed that their relatives went through hard lives because of too closely spaced pregnancies.” 244 Social & Rural Research Institute 2009 Social “Fear of social disapproval [is a barrier to adoption of spacing].” 4 Kiluvia 1991 Women’s responsibility “Communication between spouses’ partners on child spacing was not necessary. Reasons varied with age and gender.[...]. Married men also frequently saw no reason to talk to their wives, because they felt the number and timing of births was solely their responsibility.” 7 Chad Ministry of Public Health 1992 “While Chadians of both sexes agreed that ideally both husband and wife should decide to use child spacing together, there was consensus that it was ultimately a woman’s responsibility to ensure that children were spaced.” 8 Van de Walle 1986 Health concerns Child health concerns “If you have a child in your arms and you become pregnant again, it is not good. The child is tired and you, who are with a belly, are tired also... the child in your womb suffers too. Everyone will suffer, because you must feed the other one in addition to this one. The one in your womb will be tired too. You know that if there is no one to take care of the oldest one you will be obliged to carry it on top of your pregnancy.” 17 Maternal health concerns “Question. When do you want the next child? Answer: Even if it takes 3 or 4 years, I will be happy. Question: Why do you want to wait? Answer: Because I am old and a lot of blood poured out of my body during my deliveries. Can the same blood flow back into my body so rapidly?” 16 Social & Rural Research Institute 2003 Knowledge and attitudes Attitudes of the women “Personal factors driven primarily by the attitude of the woman to self and spacing.” 4 Knowledge “Ignorance of methods available and negligent attitude towards the concept of spacing per se that is catapulted either by the negative word of mouth or bad personal experiences.” Dehne 2003 Local concepts and practices Abstinence norms “Women adhering to Gurmance traditional religion reported either 24 or 36 months’ taboos, while Muslims reported much shorter norms. Most Hamallists (and the few Wahabiya) women reported a 40-day norm as prescribed in the Koran, while many ‘moderate’ Muslims reported intermediate norms of 2–5 months.” 60 Van de Walle 1986 Afraid of being mocked “As to the women, they crave for a child after 2 years, they are afraid of being mocked because they are finished with childbearing.” 28 Adeokun 1981 Agbon (female body odor) “The condition called agbon or female body odor after parturition. Once the period of agbon is over, sexual activity is commenced. The interbirth interval is thus a function of the length of agbon, the length of postpartum amenorrhoea, and the practice of pregnancy prevention after agbon has ended.” 14 Dehne 2003 Amulets, talismans, and cords “The use of amulets, talismans, and cords […] for instance in cases where women felt unprotected against an early pregnancy or guilty after having resumed sexual relations shortly after a preceding birth.” 60 Adeokun 1981 Apa (conflict between children) “The other Ikale strategy starts from the notion of apa, that is, the principle that if a surviving child is less than 1 year old and another pregnancy occurs, a conflict situation arises between the survival of the nursing child and the survival of the foetus. In order to avoid the conflict, the marital sexual relationship of the parents may be organized in such a way as to prevent the 14 Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 12 of 17 Table 3 Factors associated with birth spacing from qualitative studies (Continued) Author and year of publication Theme Factor Quotation Page undesirable pregnancy.” Millard 1984 Coitus interruptus “Conjugal disruption is more frequent in Tepetlaoxtoc and may be a contributing factor; possibly conscious efforts are more often made to birth extend intervals through coitus interruptus.” 7 De Vera 2007 Hilots (abortion for birth spacing) “Most participants said that abortion was common and a de facto method for spacing births. Because abortion is illegal in the Philippines, women sought hilots (unlicensed lay abortionists, midwives, and masseuses) to perform an abortion.” 243 Kiluvia 1991 Honoured tradition “We were strongly advised by our parents to take care of the baby for 2 years, then from here you can start thinking about having another baby. We were strictly warned of having too closely spaced children.” 17 Chad Ministry of Public Health 1992 “Child spacing was seen as a tradition that protected children’s and women’s health.” ii Lovel 1983 Local knowledge “These results show that in a traditional society the health benefits of birth spacing to mother and child are clearly well-known.” 162 Dehne 2003 Marabu “When this was becoming difficult, because my husband no longer agreed to abstain, I went to see a Marabu who gave me an amulet.” 60 Dean 1994 Polygamy “The influence of the post-partum abstinence period on birth interval and fer- tility is important. As outlined, men traditionally visit several wives, living far apart, each for a few months at a time, and this helps to sustain the post- partum abstinence period.” 1581 Ratat (traditional abstinence) “Results of this study found that there was a traditional form of contraception throughout the district, called ratat or rotow in most areas.” Lovel 1983 Social taboo “More than a quarter of the families (rural 26%, semiurban 30%) said that in their parents’ generation people were ashamed of having children too close together because it was not accepted in the culture.” 161 Van de Walle 1986 “Ah, two years [between children] is good. Certain women don’t reach 2 years. Certain women whose child is not yet walking become pregnant. I cannot understand that... If you have a child in your arms, and become pregnant, don’t you know that you are humiliated [loose face]?” 16 Kiluvia 1991 “Many of the older women felt that engaging in sexual intercourse while the mother is still breastfeeding is taboo.” 18 Adeokun 1982 Traditional medicine “The other half depends on the use of herbs and traditional devices in the prevention of pregnancy.” 11 De Vera 2007 “Five of the seven women admitted using some medicinal herbs and roots to stop pregnancy or stimulate menstruation.” 243 Kiluvia 1991 “Young men also knew few details of how traditional methods work, but they had heard about the traditional use of herbs and abstinence for birth spacing. [...] Among the most frequently mentioned methods were abstinence; “pigi” and “fungo,” in which a traditional healer ties twigs from a special tree or a “medicated” piece of cloth around a woman’s waist; herbal potions; and douching.” 9 De Vera 2007 Modernization Family planning education “The health center teaches the use of the modern methods of contraception such as pills and IUD.” 244 Social & Rural Research Institute 2005 Institutional infrastructure “Institutional infrastructure provided to the populace has also played a decisive role in adoption of spacing methods.” 4 Dehne 2003 Loss of culture “Many changes have occurred in local customs. Today, the women do not abstain for as long as we used to do.” 61 Dean 1994 “Other major factors thought to cause a decrease in child spacing were the loss of tradition generally, the loss of ratat and the loss of the post-partum ab- stinence period.” 1581 Men stay at home more “With the general degeneration of the traditional society the “men staying at home more” is likely to result in the decrease of the post-partum abstinence period.” Fig. 8, page 1582 More food “When asked specifically what women thought had caused the decrease in birth interval they gave several reasons over and above those relating to traditions, the most important of which was increase in food availability.” 1578 More health services “More health services [cause a decrease in child spacing].” 1579 More hygiene “More hygiene [causes a decrease in child spacing].” Fig. 4, page 1579 Dehne 2003 Religion “In former times, couples waited for one to two year(s) before resuming sexual contacts. Now the waiting period is 2 months or even 40 days.[...] These changes have all occurred because of the increasing influence of religion. Many people listen to the Marabu now and attend Koranic schools.” 60 & 61 Dean 1994 Other Less drought “Less drought [causes a decrease in child spacing].” Fig. 4, page 1579Less war “Less war [causes a decrease in child spacing].” Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 13 of 17 A better understanding of the factors associated with short breastfeeding is needed. Our review did not aim to identify factors associated with a shorter duration of breastfeeding; this issue should be addressed in future research. Preference for a son is widespread in North Africa, East and South Asia, and the Middle East [31]. In these regions, people reportedly prefer sons because they earn more, they perpetuate the family line, and they are recip- ients of the family inheritance [32]. One of the included studies [8] commented that Ethiopian families see a son as an economic asset. Studies reported mixed results for associations with a negative outcome of the previous pregnancy, age of the mother, education, and socioeconomic level, although with a preponderance of findings in one direction in each case. Researchers conducting the studies in this re- view often reported an association between adverse out- come of the previous pregnancy, including abortion, pregnancy loss, and multiple birth, and short birth inter- val. The author of one of the included studies [15] sug- gested that an adverse outcome might influence women to hurry into the next pregnancy without fully recover- ing from the last pregnancy. Early researchers on child mortality and fertility described this phenomenon as re- placement: “replacement would be the response to expe- rienced mortality [ …] If children die very young and the mother can have another child, the same life cycle can be approximated by replacement.” [33] p164 Most studies found more short birth interval with a younger age of the mother. Younger women are gener- ally more fertile and more sexually active; very young women are likely to be economically disadvantaged, and may have less access to and use of modern contracep- tion, which can explain this association [34–36], though the way this plays out will depend on the local context of each region. Older women are likely to have reached their desired family size and are less fertile, and therefore prone to prolong birth intervals [37, 38]. Most studies reported that less-educated women have more short birth interval. Hailu [8] explains that edu- cated women have a better-informed decision-making process, have greater autonomy, and use higher quality health care services. One study, however, reported op- posite results. Highly educated women tend to delay their first pregnancy [39] and RamaRao et al. [37] hy- pothesized that educated women may want to compress motherhood into fewer years and therefore are likely to have shorter birth intervals. Most studies in our review found more short birth interval among economically underprivileged women. Hailu [8] suggests that wealthier women have better ac- cess to health care information, services, and supplies and therefore, can apply modern contraception to pro- long birth intervals. Similarly, most studies reported expected associations between longer birth intervals with modern contracep- tion use. One cross-sectional study based on DHS sur- veys reported opposite results [40] and the authors suggested this association between short birth interval and modern contraceptive use could have been due to temporality bias, such that women who had experienced an unintended short birth interval were then preferen- tially motivated to use modern contraception. Some of the studies in this review failed to find signifi- cant associations between age of the mother, education, contraception method, marital status, occupation, and place of residence, and short birth interval. Together with the fact that some studies showed associations in the opposite direction to the prevalent findings, this sug- gests that associations with factors such as age and edu- cation of the mother are not universal and may be highly dependent on the particular context. Qualitative findings Qualitative studies explored the perceived effects of local concepts and practices not considered in quantitative studies. For example, in Burkina Faso [41] and Kenya [42], studies reported loss of traditional concepts and practices that prolong birth intervals, as a result of modernization. These qualitative findings could help to explain some associations in quantitative studies. For ex- ample, it is possible the associations between higher edu- cation and higher socioeconomic level and short birth interval could be partly explained by loss of traditions, as Table 3 Factors associated with birth spacing from qualitative studies (Continued) Author and year of publication Theme Factor Quotation Page De Vera 2007 Religion Catholicism “However, one mother stated that because of the religious saying, “children are gifts from God,” couples end up having many children because they do not have a choice but to accept them.” 243 Adeokun 1982 God’s will “Chance or God’s will in the avoidance of an inconvenient pregnancy.” 10 Social & Rural Research Institute 2008 Hinduism “Religious prohibitions dictated by certain scriptures have led to believers not subscribing to spacing.” 4 Chad Ministry of Public Health 1992 Religion “Religion was also used as a reason by some men, in both Sarh and N’Djamena, for not practicing birth spacing because children were a gift from God.” v Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 14 of 17 more privileged women may report higher levels of ac- culturation [43]. Qualitative studies considered other factors that were not examined in quantitative studies. Participants in qualitative studies mentioned that communication be- tween couples and families, the influence of local media and the society, and observation of other parents could influence birth intervals. None of the included quantita- tive studies attempted to explore associations between these factors and birth interval. Quantitative studies reported a consistent association between longer breastfeeding and longer birth interval. However, participants in one qualitative study ques- tioned the effectiveness of breastfeeding as a means to prolong birth interval, presumably based on their own experiences [44]. The WHO cautions that breastfeeding as the sole method of contraception does not fully pro- tect against new pregnancies; 5–10% of women with lac- tation amenorrhea nevertheless become pregnant [45]. Strengths and limitations Strengths of our study include a broad search strategy guided by a librarian, inclusion of five languages, no year limit, and inclusion of quantitative, qualitative and mixed-methods research, and grey literature. We chose not to conduct a meta-analysis due to con- siderable heterogeneity of study design, study quality, population characteristics, and outcome and exposure definitions. We noted the concerns of Egger and co- authors [46] that (i) residual confounding and selection bias are common in observational studies; (ii) meta- analyses of observational data may yield precise but spurious results; (iii) quantitative synthesis should not be a prominent component of systematic reviews of obser- vational studies. Although commonly performed, meta- analysis of observational studies has been criticized and some authors have suggested that this practice should be abandoned [47]. As with all systematic reviews, our results and conclu- sions are limited by the quality of the original studies. Almost all the included quantitative studies were obser- vational, with concerns about residual confounding even after multivariate analyses [46], and temporality [48] bias, not knowing if the exposure preceded the outcome. The variability of the definition of short birth interval (24, 33, and 36months) limits the conclusions of our study. The current WHO recommendation of an opti- mal birth interval [2] is based on a large body of obser- vational studies published before 2006, most of them coming from low- and middle-income countries. In other settings such as developed countries, a birth inter- val between 24 to 33months may not be considered short [49]. Conclusions We found two factors consistently associated with short birth interval: shorter breastfeeding duration and a fe- male previous child. Promotion of breastfeeding could help to reduce short birth interval, and has many other benefits [50]. Addressing the preference for a male child is a complex and longer-term challenge. The quantitative and qualitative studies yielded differ- ent and complementary findings. This highlights the po- tential value of mixed-methods research. Quantitative researchers should look for ways to investigate factors such as local knowledge and practices, modernization, and communication. Future research should use longitu- dinal and experimental designs, aim for consistency in outcome and exposure definitions, and include Latin American countries. Supplementary information Supplementary information accompanies this paper at https://doi.org/10. 1186/s12884-020-2852-z. Additional file 1. PRISMA 2009 Checklist. Filled PRISMA checklist showing 27 items and the page reporting each item. Additional file 2. Search strategy. Key terms specific to each database, Boolean operators, and truncators. Additional file 3. Full references of the records included in our study. List showing the full references of the documents included in our study. Additional file 4. Studies reporting HR. Table showing the findings of studies reporting HR. Additional file 5. Quality assessment of the studies. Table showing the quality appraisal scores (two reviewers and average). Abbreviations DHS: Demographic and Health Survey; LAM: Lactational Amenorrhoea Method; MMAT: Mixed Methods Appraisal Tool; PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses; USAID: United States Agency for International Development; WHO: World Health Organization Acknowledgements Genevieve Gore provided advice for conducting the systematic literature search. Staff at POPLINE facilitated the retrieval of grey literature. Cassandra Laurie helped proofread the final version of the manuscript and supported its write-up. Authors’ contributions JP, UA, and AC designed the study; JP and UA screened the publications and extracted the data; JP, UA and AC analyzed the data and drafted the manuscript. KO, YG, MCB, and NA participated in interpreting the findings and drafting the manuscript. All authors read and approved the final manuscript. Authors’ information Juan Pimentel is a Colombian public health physician and epidemiologist. Currently, he is a lecturer in family medicine and public health at La Sabana University, a research associate at the Research Group on Traditional Health Systems (El Rosario University, Colombia), and the head of Medical Education at the Center for Intercultural Medical Studies (Colombian NGO). He is now pursuing a Ph.D. in Family Medicine to foster cultural safety in research and clinical practice through transformative learning in medical education. Funding This work was carried out with the aid of a grant from the Innovating for Maternal and Child Health in Africa initiative, a partnership of Global Affairs Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 15 of 17 https://doi.org/10.1186/s12884-020-2852-z https://doi.org/10.1186/s12884-020-2852-z Canada (GAC), the Canadian Institutes of Health Research (CIHR) and Canada’s International Development Research Centre (IDRC). This did not influence the design and execution of the study. Availability of data and materials The datasets used and/or analysed during the current study are included within the article and its additional files. Additional information is available from the corresponding author on reasonable request. Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Competing interests The authors declare that they have no competing interests. Author details 1CIET/PRAM, Department of Family Medicine, McGill University, 5858 Chemin de la Côte-des-Neiges 3rd Floor, Suite 300, Montreal, Quebec H3S 1Z1, Canada. 2Facultad de Medicina, Universidad de La Sabana, Campus Universitario puente del común, Chía, Colombia, CP 250001. 3Escuela de Medicina y Ciencias de la Salud, Universidad del Rosario, Carrera 24 # 63 C 69, Bogotá, Colombia. 4Centro de Investigación de Enfermedades Tropicales (CIET), Universidad Autónoma de Guerrero, Calle Pino s/n Colonia El Roble, 39640 Acapulco, Guerrero, Mexico. 5Federation of Muslim Women Association of Nigeria (FOMWAN), Bauchi, Nigeria. Received: 20 November 2019 Accepted: 28 February 2020 References 1. Conde-Agudelo A, Rosas-Bermúdez A, Kafury-Goeta AC. Birth spacing and risk of adverse perinatal outcomes. JAMA. 2006;295:1809. https://doi.org/10. 1001/jama.295.15.1809. 2. World Health Organization. Report of a WHO technical consultation on birth spacing. 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Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Pimentel et al. BMC Pregnancy and Childbirth (2020) 20:156 Page 17 of 17 https://doi.org/10.1186/1471-2393-11-38 https://doi.org/10.1186/1471-2393-11-38 https://doi.org/10.1363/4017614 http://pdf.usaid.gov/pdf_docs/PNADG133.pdf https://doi.org/10.2307/2061864 https://doi.org/10.2307/2061864 https://www.pewsocialtrends.org/2015/01/14/women-and-leadership/ https://www.pewsocialtrends.org/2015/01/14/women-and-leadership/ https://doi.org/10.1016/j.jep.2009.02.035 https://doi.org/10.1016/j.jep.2009.02.035 https://searchworks.stanford.edu/view/1251451 https://doi.org/10.1136/bmj.316.7125.140 https://doi.org/10.1136/bmj.316.7125.140 Abstract Background Methods Results Conclusions Trial registration Background Methods Search strategy Eligibility criteria Data collection Methodological quality Results Studies reporting OR/RR Studies reporting HR Qualitative and mixed-methods studies Meta-analysis Quality assessment Discussion Quantitative findings Qualitative findings Strengths and limitations Conclusions Supplementary information Abbreviations Acknowledgements Authors’ contributions Authors’ information Funding Availability of data and materials Ethics approval and consent to participate Consent for publication Competing interests Author details References Publisher’s Note