Short Interpregnancy Intervals: Results from the First Baby Study

Short interpregnancy interval (SIPI), defined as 18 months or fewer between delivery and subsequent conception, has become an independent marker of maternal and child health.

We performed a secondary analysis of 18 months of data from The First Baby Study, a prospective cohort of women followed from pregnancy through 3 years after their first birth. Women with SIPIs by 6, 6 to 12, and 12 to 18 months were compared with those without conceptions at those times. We then analyzed pregnancy intention of the subpopulation of women with a SIPI of 18 months or fewer. Logistic regression analyses determined associations between maternal characteristics, including sociodemographic and reproductive indicators, and SIPI incidence and intention.

Of 3,006 participants, 795 (26.5%) had a repeat pregnancy within 18 months: 58 (1.9%) occurred within 6 months, 242 (8.1%) between 6 and 12 months, and 495 (16.5%) between 12 and 18 months. Incidence of SIPI at each interval differed by maternal characteristics, including income, marital status, and intention. Most women (84%) with a SIPI of 6 months or less classified them as unintended. Less than 2% of women with SIPIs of 18 months or fewer reported any contraceptive use in the postpartum period and no pregnancies occurred with the use of very effective methods, including long-acting reversible contraception.

The population of women at risk for SIPI is not homogenous. Among those with SIPIs, there is a stark contrast in intention between those who conceive early (≤6 months) versus later (≥12 months). Given that almost no pregnancies occurred when women used postpartum contraception, contraceptive counseling and unfettered access should be available for those at greatest risk for an early, repeat, unintended pregnancy.