Maternal Age Differences in Interpregnancy Interval and Preterm Birth Associations Accounting for Multiple...

People who recently gave birth are strongly advised to wait 6 months before attempting pregnancy. Interpregnancy intervals (IPI) of ≥ 18 months are considered optimal. Current guidance is not tailored based on maternal characteristics (e.g., age).

We evaluated whether maternal age modifies IPI-preterm birth (PTB) associations.

From a US retrospective cohort of multiparae (1997-2011), we categorised IPI: < 6, 6-11, 12-17, 18-23 (reference), 24-59 or ≥ 60 months. PTB occurred before 37 0/7 weeks' gestation. We estimated risk ratios (RR) between IPI and PTB using modified Poisson regression, adjusted for potential confounders and stratified by age at prior delivery: < 25 (n = 2484), 25-29 (n = 1626) or ≥ 30 (n = 1209) years. We conducted quantitative bias analysis to adjust for volunteer bias and dependent misclassification between IPI and gestational length (since both are calculated using the estimated start of pregnancy). We computed E-values when RR lower bounds of the 95% simulation intervals were > 1.00.

Estimates were imprecise due to small numbers. However, in terms of general patterns, PTB risk was highest with < 6 months IPI in all age groups (covariate-adjusted RR point estimates ≥ 1.30). The strongest associations were observed among 25-29 years. For ≥ 30 years, PTB risk was lowest with 6-17 months IPI. After multiple bias adjustments, estimates tended to move downward, but similar patterns remained. For 25-29 years, the lower bound of the 95% simulation interval for < 6 versus 18-23 months IPI was > 1.00, with an E-value of 3.82, suggesting unmeasured confounding would need to be very strong to explain the association.

Estimates were imprecise. However, our study adds to growing evidence that IPI associations may be weaker among older individuals. Older individuals with shorter IPI may have lower PTB risk than those with currently recommended IPI, but more research is needed.