Average maternal age was 29.9 years in the 2001 cohort (n = 47,015) and 31.4 years in the 2010 cohort (n = 38,554), which reflects the secular trend toward later childbirth. In Table 1 and S1 Table, we present the distributions of background information in the 2001 and 2010 cohorts by maternal age groups: 25.0–29.9, 30.0–34.9, 35.0–39.9, and >40.0 years. As expected, preterm births and low birthweight were more common among the advanced maternal age groups in both cohorts. The proportions of preterm birth and low birthweight were slightly higher in the <25.0 years group compared to the 25.0–29.9 group in both cohorts, which is likely because of disadvantaged socioeconomic conditions . However, government programs such as national health care coverage to support pregnant women may have been protective against adverse birth outcomes among the youngest mothers in Japan. We also observed that educational attainment and family income tended to be higher among older age groups, but full-time employment was more common among younger mothers. This reflects the pattern of labor force participation among Japanese women, who typically leave work once they begin childbearing . As shown in Figs 1 and 2, we observed some drop-out between the first and subsequent surveys. Those who were lost to follow-up tended to be younger in maternal age with low parental education, maternal smoking habits, and lower household income.