Interpregnancy interval and adverse live‐birth outcomes after a non‐live‐birth prior pregnancy: A 2024 U.S. population‐based analysis

OBJECTIVE: To examine whether the association between interpregnancy interval and adverse live-birth outcomes differs by the outcome of the preceding pregnancy. METHODS: We conducted a retrospective population-based cohort study of singleton live births in the 50 U.S. states and the District of Columbia in 2024 (final Natality Public Use File) with at least one previous pregnancy and a classifiable most recent prior pregnancy outcome (live birth vs other outcome). Derived interpregnancy interval was categorized as <6, 6-11, 12-17, 18-23 (reference), 24-59, and ≥60 months. The primary outcome was preterm birth (<37 weeks); secondary outcomes were low birth weight, very low birth weight, neonatal intensive care unit (NICU) admission, and a composite adverse outcome. Modified Poisson regression with robust variance estimated adjusted risk ratios (aRRs) and tested the interval × prior-outcome interaction. RESULTS: Of 1 995 754 births, 420 882 (21.1%) followed a non-live-birth outcome. The association between interval and preterm birth differed by prior outcome (interaction P < 0.001). After a prior live birth, risk was U-shaped (<6 months: aRR 1.61, 95% confidence interval [CI]: 1.57-1.66; ≥60 months: 1.30, 1.27-1.33). After another outcome, intervals <6 months were not associated with increased risk (aRR 0.90, 0.87-0.94), although risk exceeded that after a live birth at every interval of 6 months or longer. Findings were similar for secondary outcomes and robust in sensitivity analyses, including adjustment for parity. CONCLUSION: The association between interpregnancy interval and adverse outcomes of the subsequent live birth depends on the outcome of the preceding pregnancy and should not be interpreted uniformly across prior pregnancy outcomes.