Recent Status of Low-Birth-Weight Infants in Japan

Objective In this study, we examined the recent trends in neonatal birth weight in Japan with the precise gestational age distinctions of 39 weeks, which is the period internationally considered to be the most optimal for delivery based on the perinatal outcomes. Materials and methods Firstly, we calculated the frequency of low-birth-weight (LBW) infants beyond 22 weeks of gestation in Japan using the birth notifications from 2000, 2010, and 2020. Secondary, we analyzed the birth weight trends of the first singleton male and female infants born to primiparous women at 39 weeks of gestation. Results The frequency of LBW infants increased significantly in 2010 compared to 2000 (odds ratio 1.13, 95% confidence interval 1.12-1.14, p < 0.01); however, it decreased significantly in 2020 (odds ratio 0.952, 95% confidence interval 0.943-0.961, p < 0.01), although it did not reach the level in 2000 (p < 0.01). The average neonatal birth weight born to primiparous women at 39 weeks of gestation in 2010 was significantly lower than in 2000 (p < 0.01); however, in 2020 it was significantly higher than in 2000 although some differences were observed between the neonatal sexes (p < 0.01). Conclusion The frequency of LBW infants has been declining from 2010 to 2020 in Japan. In addition, the average neonatal birth weight born to primiparous women at 39 weeks of gestation in 2020 was significantly higher than in 2010.