Interpregnancy interval after live birth or pregnancy termination and estimated risk of preterm birth: a retrospective cohort study.

ABSTRACT

OBJECTIVES: We assessed whether interpregnancy interval (IPI) length after live birth and after pregnancy termination was associated with preterm birth (PTB). DESIGN: Multiyear birth cohort. SETTINGS Fetal death, birth and infant death certificates in California merged with Office of Statewide Health Planning and Development. POPULATION One million California live births (2007-10) after live birth and after pregnancy termination. METHODS: Logistic regression was used to estimate odds ratios (ORs) of PTB of 20-36 weeks of gestation and its subcategories for IPIs after a live birth and after a pregnancy termination. We used conditional logistic regression (two IPIs/mother) to investigate associations within mothers. MAIN OUTCOME MEASURE: PTB relative to gestations of ≥ 37 weeks. RESULTS: Analyses included 971 211 women with IPI after live birth, and 138 405 women with IPI after pregnancy termination with 30.6% and 74.6% having intervals of <18 months, respectively. IPIs of <6 months or 6-11 months after live birth showed increased odds of PTB adjusted ORs for PTB of 1.71 (95% CI 1.65-1.78) and 1.20 (95% CI 1.16-1.24), respectively compared with intervals of 18-23 months. An IPI >36 months (versus 18-23 months) was associated with increased odds for PTB. Short IPI after pregnancy termination showed a decreased OR of 0.87 (95% CI 0.81-0.94). The within-mother analysis showed the association of increased odds of PTB for short IPI, but not for long IPI. CONCLUSIONS: Women with IPI <1 or >3 years after a live birth were at increased odds of PTB-an important group for intervention to reduce PTB. Short IPI after pregnancy termination was associated with reduced odds for PTB and needs to be further explored. TWEETABLE ABSTRACT Short and long IPI after live birth, but not after pregnancy termination, showed increased odds for PTB.