Influence of interpregnancy interval on birth timing.

Area of Science:

• Reproductive health
• Maternal-fetal medicine
• Perinatology

Background:

• Inadequate birth spacing, defined as short interpregnancy intervals (IPIs), is a known risk factor for adverse pregnancy outcomes.
• Previous research has primarily focused on the association between short IPIs and preterm birth.

Purpose of the Study:

• To evaluate the impact of insufficient birth spacing on the distribution of birth timing throughout gestation.
• To determine if short IPIs influence the likelihood of delivering at specific gestational weeks.

Main Methods:

• A population-based retrospective cohort study utilizing vital statistics birth records from Ohio, USA (2006-2011).
• Analysis included singleton, non-anomalous live births (≥20 weeks) to multiparous mothers.
• Birth frequency at each gestational week was compared for short IPIs (<6, 6-12, 12-18 months) against a normal IPI (≥18 months) referent group.

Main Results:

• The study analyzed 454,716 births; 87% had normal IPIs (≥18 months), 10.7% had IPIs of 12-18 months, and 2.2% had IPIs <12 months.
• Short IPIs (<12 months) were associated with a significantly higher risk of delivery before 39 weeks (adjusted OR 2.78).
• Women with IPIs <12 months had 53.3% of births before 39 weeks vs. 37.5% with normal IPIs (P<0.001), and fewer births at ≥40 weeks (16.9% vs. 23.2%).

Conclusions:

• Inadequate birth spacing, particularly short IPIs, is associated with a reduction in gestational age across all births, not just preterm births.
• Pregnancies following short IPIs demonstrate a higher frequency of birth before 39 weeks and a lower frequency at 40 weeks or beyond.
• This results in an overall shift towards earlier delivery and shortened pregnancy duration when interpregnancy intervals are inadequate.