Decreasing Full-term Neonatal Birthweight over the Past 2 Decades in a Single Institute in Japan

Aim: The aim of this study was to investigate whether full-term neonatal birthweight is decreasing or not.

Material And Methods: Subjects were selected from the registration database of full-term singletons from a single local practitioner at two time-points, 1991 (n = 750) and 2011 (n = 442). Maternal age, parity, and smoking habits were recorded. Baseline characteristics included pre-pregnancy bodyweight (BW), body mass index (BMI), bodyweight before parturition, and weight gain during pregnancy. Neonatal birthweight and height were recorded. Variables correlated with neonatal size were investigated.

Results: Male birthweight decreased from 3248 ± 409 (n = 373) to 3149 ± 430 g (n = 230) (P < 0.001). Female birthweight also decreased from 3148 ± 378 (n = 377) to 3063 ± 343 g (n = 212) (P < 0.01). Gestational age was shorter (40.1 ± 1.1 to 39.6 ± 1.1 weeks, P < 0.001). Pre-pregnancy maternal BMI did not change (20.7 ± 2.6, n = 750 to 20.5 ± 2.7, n = 442). Weight gain during pregnancy decreased (11.3 ± 3.8 to 10.8 ± 3.4 kg, P < 0.05) and was positively correlated with birthweight (P < 0.001). On multiple regression analysis, time was inversely correlated with birthweight, while it was positively correlated with pre-pregnancy BMI, weight gain during pregnancy, and gestational age.

Conclusions: In addition to maternal slenderness, increasing lower weight gain during pregnancy and medical intervention by physicians may in part contribute to the decrease in full-term birthweight over the past 2 decades.