The probability of preterm or early term second live births in the southern U.S. State of Georgia, 2011-2020.

Background: The extent to which past preterm or early term births are risk factors for recurrent shortened gestational age at delivery in the US is unclear. The underlying causes of shortened gestational age and the role of maternal health can vary over time and across the 50 US states.

Objectives: To estimate differences in the probability of a second live birth being full term (> 38 weeks' gestation), early term (37-38 weeks') or preterm (< 37 weeks') conditional on the gestational age of the first live birth.

Study Design: We used linked birth and hospital discharge records from the state of Georgia to construct a retrospective cohort of individuals whose first and second births resulted in a singleton live birth (2011-2020). Multinomial models were used to estimate the difference in the probability of the second live birth gestational age category (< 32, 32-36, 37-38, ≥ 39 weeks) conditional on the first birth gestational age category. Baseline models were only adjusted for year fixed effects. Subsequent models were adjusted for birthing individual characteristics: race, age at first birth, ethnicity, country of birth, and education at first birth; second birth health, behavioral and socioeconomic risk factors; and, for interpregnancy birth interval and change in paternal characteristics between first and second births. All analyses were completed in Stata 17.

Results: Individuals whose first live birth was preterm or early term were significantly less likely to have a second full term live birth, compared to individuals who had a first full term live birth. The probability of a full term second live birth following a first preterm live birth at < 32 weeks or 32-36 weeks decreased by 27.7% points (pp) (95% CI -30.0, -25.2) and 22.1 pp (95% CI-23.3, -21.0) respectively. Similarly, the probability of a full term second birth following a first early term birth decreased by 14.9 pp (95% CI - 15.7, -14.2). Individuals who had early term or preterm first births were more likely to have early term and preterm second births, with higher risk for recurrent preterm birth among birthing individuals with earlier preterm first births. For example, following a first birth that is early term, the probability of a second birth at 32-36 weeks increased by 4.7 pp (95% CI 4.2, 5.1), whereas following a first birth at < 32 weeks the probability of a second birth at 32-36 weeks increased by 13.6 pp (95% CI 11.5, 15.6).

Conclusions: A first live birth that is early term or preterm predicts a higher probability of a second early term or preterm birth. Health information systems that flag these early birth outcomes on the problem list may help clinicians address known risk factors interconceptionally and prenatally in a subsequent pregnancy.