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The optimal range of gestational weight gain (GWG) was recently raised in Japan. This may help reduce small-for-gestational-age (SGA) infants, but may also increase large-for-gestational-age (LGA) infants. This study performed hypothetical experiments to determine effective GWG advice based on quantile regression analysis. In a total of 354,401 singleton pregnancies registered in the perinatal database of the Japan Society of Obstetrics and Gynecology (2013-2017), the proportions of SGA and LGA were 9.33% and 11.13%, respectively. Using regression coefficients of GWG across the birth weight-for-gestational-age quantile distribution, we analyzed changes in their proportions by simulating a uniform 3-kg extra increase in GWG or an increase or decrease based on GWG adequacy. A hypothetical experiment of a uniform increase in GWG resulted in SGA and LGA proportions of 7.26% (95% confidence interval 7.15-7.36) and 14.51% (14.37-14.66), respectively. By contrast, assuming a 3-kg increase in women with inadequate GWG and a 3-kg decrease in women with excessive GWG resulted in SGA and LGA proportions of 8.42% (8.31-8.54) and 11.50% (11.37-11.62), respectively. Our real-world data analysis suggests that careful adjustment of GWG based on GWG adequacy will be effective in optimizing infant birth weight in Japan., (© 2023. The Author(s).)

Over 80% of perinatal mortality in Japan is due to stillbirths after 22 weeks of gestation, with one in 300 families experiencing fetal loss every year. This study aimed to assess causes and risk factors for singleton stillbirth in Japan. A retrospective cross-sectional study was conducted using the Japan Society of Obstetrics and Gynecology Perinatal Database from January 2013 to December 2014. A total of 379,211 births including 2,133 stillbirths were analyzed. Causes of death were classified into eight categories. A multi-level Poisson regression model was used to assess the relationship between stillbirth and key covariates. Causes of death were unknown in 25-40% of stillbirths across gestational age. Placental abnormality accounted for the largest proportion of known causes, followed by umbilical cord abnormality. Stillbirth risk was increased among small-for-gestational-age infants (adjusted relative risk [ARR]: 3.78, 95% confidence interval [CI]: 3.31-4.32) and nulliparous women (ARR: 1.19, 95% CI: 1.05-1.35). Maternal underweight, pregnancy-induced hypertension and oligohydramnios showed a protective effect. Our finding suggests that stillbirths occurring among women with known complications are likely already being prevented. Further reduction in stillbirths must target small-sized fetuses and nulliparous women. Improved recording of the causal pathways of stillbirths is also needed.