Pre-pregnancy body mass index as a predictor of low birth weight infants in Japan.

BACKGROUND AND OBJECTIVES The prevalence of low birth weight (LBW) infants in Japan has doubled in the last several decades. The objective of this study was to examine the effects of pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) on LBW infants of Japanese women. METHODS AND STUDY DESIGN This retrospective study was conducted using data on 1,336 mothers (mean age, 34.0 years)whose pre-pregnancy BMI was less than 23 kg/m2 and their singleton infants were born at full term between January and December in 2011. The outcome of interest was LBW infants (less than 2,500 g). The main exposure variables were pre-pregnancy BMI and GWG. The effects of these two variables on LBW were determined after adjusting for confounder variables such as maternal age, smoking, drinking, parity, gestational week at birth and infant gender. RESULTS The proportion of LBW infants was 4.2% in total, 6.1% among underweight mothers (<18.5 kg/m2) and 3.5% among normal weight mothers (18.5-22.9 kg/m2).A stepwise multivariable logistic regression model showed that underweight mother were more likely [odds ratio (OR) 1.86, 95% confidence interval (CI), 1.04-3.31] than normal weight mother to deliver a LBW infant. Mothers with inadequate GWG <8.5 kg were more likely to deliver a LBW infant (OR 1.66, 95% CI: 0.80-3.45) compared with mothers who gained 10.5-12.4 kg (the third lowest quartile) but this did not reach statistical significance. CONCLUSIONS This study demonstrated that mothers who were underweight before pregnancy were independently associated with the delivery of LBW infants.