Factors Associated with Post-term Birth and its Relationship to Neonatal Mortality in Japan: An Analysis of National Data From 2017 to 2022.

Objectives: Prior research has not yet examined the relationship between post-term birth and neonatal mortality in Japan, along with factors associated with post-term birth. We investigated these associations utilizing nationwide birth data from Japan.

Methods: Birth and mortality data were obtained from the Vital Statistics of Japan for the years 2017 to 2022. The post-term birth rate was calculated by birth characteristics, and the neonatal mortality rates for post-term and term births were computed. Additionally, log-binomial regression analysis was employed to explore the associations between post-term birth and neonatal mortality, as well as between various characteristics and post-term birth. The characteristics considered included infant sex, maternal age group, parity, maternal nationality, maternal marital status, and household occupation.

Results: This study analyzed data from 4,698,905 singleton infants born at 37 weeks of gestational age or later. Regression analysis revealed that post-term birth was positively associated with neonatal mortality. The adjusted risk ratio for neonatal mortality in post-term compared to term births was 8.07 (95% confidence interval, 5.06 to 12.86). Factors positively associated with post-term birth included female infant sex, older maternal age, primiparity, non-Japanese maternal nationality, unmarried status, and various household occupations, including farmer, full-time worker at a smaller company, other type of worker, and unemployed. Younger maternal age was inversely associated with post-term birth.

Conclusions: In Japan, post-term birth represents a risk factor for neonatal mortality. Additionally, sociodemographic characteristics, such as maternal marital status, nationality, and parity were found to be predictors of post-term birth.