Birth spacing in the United States—Towards evidence‐based recommendations

Reducing the prevalence of short pregnancy spacing following a live birth is a well-established public health goal in the United States.1 This goal is, in part, based upon the wealth of studies that show that short interpregnancy interval is associated with higher risk of adverse subsequent pregnancy outcomes such as preterm birth. Unlike many other risk factors for adverse pregnancy outcomes, for example, socio-economic position or race/ethnicity, the length of time between a live birth and a subsequent pregnancy is modifiable—through improved access to contraceptive counselling and services—making it an attractive target for public health intervention. The extent to which these previously-observed associations should be used to derive birth spacing policies for US women, however, remains unclear. The most widely hypothesised causal mechanism posits that short birth spacing does not allow for full repletion of maternal nutritional status before the next pregnancy begins, which increases the risk of adverse pregnancy outcomes. However, there are few interventional studies showing that reducing short birth spacing leads to better pregnancy outcomes and none showing higher rates of nutritional repletion as the causal mechanism. Further, much of the observational literature comes from low- or middle-income countries where maternal nutritional status and fertility patterns are different from those in the US. For example, compared with low- and middle-income countries, breast-feeding initiation, and prevalence of maternal malnutrition are lower in the US, indicating that having a live birth may be less nutritionally taxing for US women compared to women from lower income countries. In addition, mean maternal age at first birth is higher and total fertility rate (births per woman) is lower. These differences suggest that worldwide recommendations for optimal birth spacing to reduce adverse outcomes from organisations such as the World Health Organization2 may not be appropriate for US women. Therefore, a fresh examination of the evidence supporting a causal relationship between short birth spacing and subsequent pregnancy outcomes for US women is needed. The purpose of this theme issue in Paediatric and Perinatal Epidemiology is to examine the effect of short interpregnancy interval—the time between delivery of a live birth and the start of the next pregnancy—on adverse pregnancy outcomes for women in the US. Herein, we provide a description of current interpregnancy intervals patterns in the US, and outline the motivation for a recent meeting convened to examine the evidence for the causal effect of short birth spacing on adverse pregnancy outcomes for the US. In addition, we briefly describe how the manuscripts included in this supplement advance our understanding of the effects of short interpregnancy interval on health outcomes, and suggest how the findings from this supplement can be used by researchers, women, and family planning providers. In 2016, approximately 29% of singleton non-first births in the US were preceded by an interpregnancy interval <18 months (4.7% <6 months, 10.7% 6-11 months, and 13.8% 12-18 months; Figure 1). National interpregnancy interval data are derived from US birth certificate information on gestational age (obstetric estimate) and date of last live birth, which was added to the US standard birth certificate as part of the 2003 revision. These data were only available from select states that had adopted the 2003 revision; however, similar proportions of births followed an interpregnancy interval <18 months in 2011 (29.6%: among 36 states and the District of Columbia [representing 83% of US births])3 and in 2014 (28.9%; among 47 states and the District of Columbia [representing 96% of US births]).4 Nationally representative data on interpregnancy intervals are also available from the National Survey of Family Growth. These data are used by Healthy People to monitor pregnancy spacing in the US.1 Among women interviewed in 2006-2008, 35.5% (95% confidence interval [CI] 31.5, 39.5) of the most recent pregnancies reported among parous women followed an interpregnancy interval <18 months, which decreased to 27.0% (95% CI 22.4, 31.6) among women interviewed in 2013-2015 (Figure 2). These intervals include all self-reported pregnancies following a live birth among women age 15-44, regardless of the subsequent pregnancy outcome (ie, includes subsequent pregnancies that end in pregnancy loss, induced abortion, or live birth) and are based on retrospective reporting of pregnancy history. These two national measures show discrepant trends over time in progress towards the goal of reducing interpregnancy intervals <18 in the US; however, both measures show that short intervals remain common, accounting for approximately 30% of all interpregnancy intervals. On September 14-15, 2017, the Office of Population Affairs convened an expert work group meeting, Birth Spacing and Adverse Pregnancy Outcomes, in Washington, DC, with the aim of critically evaluating the evidence for the causal effect of short interpregnancy intervals on adverse perinatal and maternal health outcomes in the US. The Office of Population Affairs was interested in convening this meeting because it administers the Title X Family Planning Program, the only federal grant programme dedicated solely to providing individuals with family planning and related preventive health services. The presumed beneficial health effect of using contraception to space births is an underlying component of the 2014 Providing quality family planning services: Recommendations of CDC and the US Office of Population Affairs,5 published by the Office of Population Affairs and the Centers for Disease Control and Prevention (CDC), which includes evidence-based recommendations aimed at family planning providers. In addition, several recent publications on interpregnancy intervals using maternally-linked pregnancy data (ie, linking women's successive pregnancies or live births) have prompted a renewed interest in examining the causal effect of short birth spacing on pregnancy outcomes.6-9 These studies have used maternally-linked pregnancy data to conduct analyses of interpregnancy intervals within individual women (ie, comparing her sibling offspring). This sibling comparison design helps to control for confounders that remain constant across a woman's pregnancies, which can be difficult to accomplish using standard approaches. These studies have led to some surprising novel findings, such as null associations between very short interpregnancy intervals (eg, <6 months) and preterm birth,6, 8 leading to speculation that much of the prior work on interpregnancy intervals and adverse outcomes may be biased due to methodologic flaws.8, 10 This supplement includes six manuscripts related to topics discussed at the Office of Population Affairs Birth Spacing and Adverse Pregnancy Outcomes meeting. The first paper is a summary of the meeting proceedings,11 including a discussion of the strength of the evidence for the causal effect of short interpregnancy intervals on adverse perinatal and maternal health outcomes and priorities for future work. The second paper discusses good practices for the future design, analysis, and interpretation of observational studies on interpregnancy intervals and adverse pregnancy outcomes identified by experts at the meeting.12 The supplement also includes two updated systematic reviews on short interpregnancy interval and perinatal13 and maternal14 health outcomes in high-income settings; preliminary results from these systematic reviews were presented at the meeting and are briefly summarised in the meeting proceedings.11 These reviews include an assessment of study quality that reflects the good practices discussed at the expert work group meeting. The paper by Thoma et al15 describes the use of interpregnancy interval data derived from US birth certificate records, and includes an overview of data quality of interpregnancy interval and relevant covariate and outcome variables. The final paper, by Liauw and colleagues16 provides an analysis of interpregnancy interval and small for gestational age birth using data from Scandinavia, controlling for pregnancy intention and enhanced information on socio-economic factors, both of which were highlighted by the expert work group as being important potential confounders. The papers resulting from the expert work group meeting Birth Spacing and Adverse Pregnancy Outcomes11-14 are most relevant to researchers conducting analyses on or interpreting findings from studies on short interpregnancy intervals. Nevertheless, US women and their providers may also find these high-level summaries of the strengths and limitations of existing evidence to be informative for their decision making regarding the timing of subsequent pregnancies and use of contraception. This information on health outcomes can be combined with other social and economic factors that are important to women and their partners when making birth spacing decisions. The other papers in this supplement15, 16 build upon the meeting-related papers, providing more depth and context, which may be used by US women and their providers when considering the applicability of the WHO birth spacing recommendations to their unique situation. The authors thank Marie Thoma, from the University of Maryland, and Gladys Martinez, from the National Center for Health Statistics, for providing special tabulations for this manuscript. No financial or other disclosures of conflict of interests were reported by the authors of this paper. KAA performed this work under employment of the US federal government. JAH is supported by a Canada Research Chair in Perinatal Population Health. The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Office of Population Affairs, Office of the Assistant Secretary for Health, Department of Health and Human Services.

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