• Birth spacing;
  • birth timing;
  • interpregnancy interval;
  • preterm birth


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.

Study Population

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.

Main outcome measures

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, adjOR (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,< 0.001. Likewise, birth at ≥40 weeks was decreased (16.9%) following short IPI <12 months 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 <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.