Influence of interpregnancy interval on birth timing

ObjectiveTo assess the influence of inadequate birth spacing on birth timing distribution across gestation. DesignPopulation-based retrospective cohort study using vital statistics birth records. SettingOhio, USA. Study PopulationSingleton, non-anomalous live births 20weeks to multiparous mothers, 2006-2011. MethodsBirth frequency at each gestational week was compared following short IPIs of <6, 6-12 and 12-18months versus referent group, normal IPI 18months. Main outcome measuresFrequency of birth at each gestational week; preterm <37weeks; <39 and 40weeks. ResultsOf 454716 births, 87% followed a normal IPI 18months, 10.7% had IPI 12-18months and 2.2% with IPI <12months. The risk of delivery <39weeks was higher following short IPI <12months, adjOR (odds ratio) 2.78 (95% CI 2.64, 2.93). 53.3% of women delivered before the 39th week after IPI <12months compared with 37.5% of women with normal IPI, P<0.001. Likewise, birth at 40weeks was decreased (16.9%) following short IPI <12months compared to normal IPI, 23.2%, adjOR 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 <12months and 12-18months compared to, birth spacing 18months. ConclusionsWhile 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 IPIs have a higher frequency of birth at all weeks of gestation prior to 39 and fewer births 40weeks, resulting in overall shortened pregnancy duration.