Influence of interpregnancy interval on birth timing

Objective To assess the influence of inadequate birth spacing on birth timing distribution across gestation. Design Population‐based retrospective cohort study using vital statistics birth records. Setting Ohio, USA . Study Population Singleton, non‐anomalous live births ≥20 weeks to multiparous mothers, 2006–2011. Methods Birth frequency at each gestational week was compared following short IPI s of <6, 6–12 and 12–18 months versus referent group, normal IPI ≥18 months. Main outcome measures Frequency of birth at each gestational week; preterm <37 weeks; <39 and ≥40 weeks. Results Of 454 716 births, 87% followed a normal IPI ≥18 months, 10.7% had IPI 12–18 months and 2.2% with IPI <12 months. The risk of delivery <39 weeks was higher following short IPI <12 months, adj OR (odds ratio) 2.78 (95% CI 2.64, 2.93). 53.3% of women delivered before the 39th week after IPI <12 months compared with 37.5% of women with normal IPI , P  < 0.001. Likewise, birth at ≥40 weeks was decreased (16.9%) following short IPI <12 months compared to normal IPI , 23.2%, adj OR 0.67 (95% CI 0.64, 0.71). This resulted in a shift of the frequency distribution curve of birth by week of gestation to the left for pregnancies following a short IPI <12 months and 12–18 months compared to, birth spacing ≥18 months. Conclusions While short IPI is a known risk factor for preterm birth, our data show that inadequate birth spacing is associated with decreased gestational age for all births. Pregnancies following short IPI s have a higher frequency of birth at all weeks of gestation prior to 39 and fewer births ≥40 weeks, resulting in overall shortened pregnancy duration.