Combination of parity and pre-pregnancy BMI and low birth weight infants among Japanese women of reproductive age.

This study aimed to examine the effects of parity and pre-pregnancy body mass index (BMI) on low birth weight (LBW) infants among Japanese women. Participants included 1,518 mothers (mean age 34.0 years) of singleton full-term infants in 2011. The incidence of LBW infants was 7.5% in primiparous women with BMI<18.5 (Group A; n=239), 4.0% in multiparous women with BMI<18.5 (Group B; n=124), 6.0% in primiparous women with 18.5≤BMI<25 (Group C; n=715), and 1.8% in multiparous women with 18.5≤BMI<25 (Group D; n=440). A multivariable logistic regression model revealed that mothers in Group A were more likely to deliver a LBW infant [odds ratio (OR) 6.41, 95% confidence interval (CI), 2.65-15.49] than were mothers in Group D. Being both underweight (OR 1.8, 95% CI: 1.05-3.11) and primiparous (OR 3.41, 95% CI: 1.82-6.44) were independently associated with LBW infants. This study demonstrated that the characteristics of primiparous and underweight in mothers are additively associated with LBW infants.