Birth spacing in the United States-Towards evidence-based recommendations.

Reducing the prevalence of short pregnancy spacing following a live birth is a wellestablished 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, socioeconomic 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 previouslyobserved 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 lowor middleincome countries where maternal nutritional status and fertility patterns are different from those in the US. For example, compared with lowand middleincome countries, breastfeeding 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.