Background: The rate of low-birth weight infants in Japan has been increasing. Being underweight and increasing maternal age are risk factors for delivering low birthweight infants. Currently, pregnant Japanese women exhibit these risk factors for delivering low birthweight infants; however, little research has been conducted. This study aimed to investigate the associations between pregnancy characteristics in Japanese women and low birth weight infants. Methods: We enrolled 2551 pregnant women at >36 weeks of gestation and who delivered in metropolitan Tokyo. Pre-pregnancy body mass index was categorized using the World Health Organization classification. Gestational weight gain and birth weight were categorized based on the guidelines of the Ministry of Health, Labour and Welfare, Japan. The differences in pregnancy characteristics among three birth-weight categories,i.e., low birth weight, normal, and macrosomia, were assessed using the chisquare test. The risk ratios of low birth weight were estimated using logistic regression analysis. Results: In Japanese pregnancies, birth weight was associated with gestational week at delivery, gestational weight gain, pre-pregnancy body mass index, sex, parity, and maternal age (P<0.05). In particular, the risk of low birth weight increased with advanced maternal age (odds ratio=1.97), insufficient gestational week at delivery (odds ratio=9.00), low gestational weight gain (odds ratio=2.76), and female infant (odds ratio=2.06). Further, it decreased in multiparous women compared with that in primiparous women (odds ratio=0.57). Conclusions: These findings suggest that pregnant Japanese women who are ≥40 years old with a gestational weight gain of <7.0 kg, <37 gestational weeks at delivery, female baby, and primiparous should be checked carefully during the gestational period. In order to decrease the risk of low birth weight babies, women should be educated early about childbearing and pregnancy.