Influence of interpregnancy interval on birth timing

To assess the influence of inadequate birth spacing on birth timing distribution across gestation.

Population-based retrospective cohort study using vital statistics birth records.

Ohio, USA.

Singleton, non-anomalous live births ≥20 weeks to multiparous mothers, 2006-2011.

Birth frequency at each gestational week was compared following short IPIs of <6, 6-12 and 12-18 months versus referent group, normal IPI≥18 months.

Frequency of birth at each gestational week; preterm <37 weeks; <39 and ≥40 weeks.

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.

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 IPIs 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.