Interactive Effects of Parental Age on Adverse Birth Outcomes in Japan: An Investigation of Nationwide Birth Data

We investigated the interactive effects of parents' ages, as well as the overall effects of paternal age, on adverse birth outcomes using nationwide data from Japan.

Live birth data from the Vital Statistics Surveys in Japan from 2010 to 2023 were analyzed. Preterm birth, low birth weight, macrosomia, small-for-gestational-age (SGA) status, and large-for-gestational-age status were defined as adverse birth outcomes. Log-binomial regression models were used to examine associations between maternal and paternal ages and adverse birth outcomes. In addition to regression analyses using maternal and paternal age groups separately, we conducted regression analyses using combined parental age groups to assess interactive effects.

Data from 12,279,842 live births were analyzed. Overall, the risk ratio (RR) increased significantly with an increase in the paternal age categories for all adverse birth outcomes. In contrast, among mothers aged ≥30 years, the RRs for preterm birth, low birth weight, and SGA status were higher for infants of fathers aged <25 years than for those of fathers aged ≥35 years. In particular, the RRs for these outcomes, as well as macrosomia, were highest for the combination of the youngest paternal age group (<25 years) and the oldest maternal age group (≥40 years).

Analysis of nationwide birth data indicated that the effects of paternal age on adverse birth outcomes varied according to maternal age, underscoring the importance of assessing pregnancy risk according to combined parental age.