Association of premarital pregnancy with adverse birth outcomes and its characteristics in Japan

Background: A study investigating the association between premarital pregnancy and the adverse birth outcomes has not been conducted in Japan. This study aimed to investigate an association of premarital pregnancy with adverse birth outcomes and its characteristics in Japan, using national birth data. Methods: Birth data from the Vital Statistics: Occupational and Industrial Aspects for the fiscal years 2010, 2015, and 2020 were used. Firstborn and singleton births were used, and we restricted the data to infants born to Japanese parents. We defined the status of premarital pregnancy based on the length of marriage at the time of birth. Rates of preterm birth, term low birth weight (TLBW), and small-for-gestational-age (SGA) were used as outcomes. Log-binomial regression analysis was conducted to calculate the adjusted risk ratio of premarital pregnancy for each of the outcomes. Furthermore, logistic regression analysis was conducted to identify factors associated with premarital pregnancy. Results: Data from 888,459 births were included in the analysis. The results of log-binomial regression showed that the risk of premarital pregnancy was statistically significantly higher than that of postmarital pregnancy for all the outcomes, and the risk ratios were 1.65 (95% confidence intervals (CI):1.58, 1.72), 1.17 (95% CI:1.12, 1.22), and 1.12 (95% CI:1.08, 1.17) for preterm birth, TLBW, and SGA, respectively. The results of logistic regression analysis showed that lower non-manual workers, manual workers, and others were significantly and positively associated with premarital pregnancy compared to upper non-manual workers in terms of maternal and paternal occupations. Conclusions: It was shown that premarital pregnancy was associated with a higher risk of preterm birth, TLBW, and SGA and was positively associated with parental occupations such as lower non-manual workers and manual workers in Japan.