Effects of Maternal Factors on Birth Weight in Japan

Objective. We investigated the possible factors related to the birth weight (BW) using the Japanese perinatal database. Methods. The live infants born at 37 to 41 weeks of gestation were enrolled in this study. Cases with diabetic pregnancy, preeclampsia, an anomalous fetus, and a fetus with chromosomal abnormalities were excluded. A multiple regression analysis for confounding factors and an analysis of covariance (ANCOVA) for comparing the BW in 2006 and 2010 were used for the statistical analysis. Results. The BW significantly decreased from 2950.8 g in 2006 (n = 27,723) to 2937.5 g in 2010 (n = 38,008) in the overall population, and this decrease was similar for male and female neonates. All confounding factors, except for the mode of delivery, affected the BW. Primiparity, smoking, and a female gender were related to the decrease in BW, whereas maternal age, maternal height, weight gain during pregnancy, BMI, the use of in vitro fertilization, induction of labor, and gestational duration were related to an increased BW. The ANCOVA showed that no significant change of the BW was seen between 2006 and 2010 (the difference was 2.164 g, P = 0.414). Conclusion. The gestational duration is the most important factor affecting the BW in singleton term infants.