Associations between interpregnancy intervals and adverse birth outcomes using nationwide birth data in Japan

Abstract

Abstract We investigated the associations between interpregnancy intervals (IPIs) and birth outcomes using nationwide birth data in Japan. The live birth data in the Vital Statistics in Japan from 2010 to 2023 were used. The IPI was calculated by identifying the same parents from their characteristics. The rates of preterm birth, low birth weight, and small-for-gestational-age (SGA) were used as birth outcomes. An analysis of overall birth data using a modified Poisson generalized estimation equation model as well as a within-mother analysis based on a matched case-control design were conducted. The IPIs of 0–5 and 6–11 months were significantly associated with a higher risk of preterm birth and low birth weight. In addition, longer IPIs were also significantly associated with a higher risk of those outcomes. In contrast, the risk ratio (RR) for SGA consistently increased with an increase in the IPI, and the RRs of 0–5 months and ≥ 60 months were the lowest and highest, respectively. The result of the within-mother analysis was similar to that of the analysis performed using overall birth data in many cases. The results indicated that the relationship between short IPIs and adverse birth outcomes differed depending on the type of outcome.