Aim: The purpose of our study was to investigate the association between low maternal prepregnancy body mass index (BMI) less than 18.5 kg/m2 and the incidence of small for gestational age (SGA) infants. Material & Methods: This was a cross-sectional study. The women with BMI of less than 25.0 kg/m2 who gave birth to single term infants (37-42 weeks) at clinics and hospitals in the Tokyo metropolitan area between 2003 and 2004 were analyzed for risk factors for SGA. Results: Five hundred and seventy-two women were underweight (BMI < 18.5 kg/m2) and 2708 (75.1%) were normal (18.5 ≤ BMI < 25.0 kg/m2). Birthweight, analyzed by multiple regression analysis, was highly related (P < 0.05) to gestational age, maternal age, parity, prepregnancy BMI, maternal weight gain and maternal smoking status. Women with a less than 9 kg weight during pregnancy were 1.8 times (confidence interval [CI], 1.6-2.2) more likely to give birth to an SGA infant compared with women who gained 9-12 kg. Maternal smoking more than 10 cigarettes per day was associated with an increased risk of having an SGA infant (odds ratio [OR], 2.5; CI, 1.8-3.5). Women with prepregnancy BMI less than 21.0 kg/m2 were associated with an increased risk of having an SGA infant (OR, 1.6; CI, 1.3-2.2 for BMI < 18.5 kg/m2, and OR, 1.4; CI, 1.2-1.7 for 18.5 ≤ BMI ≤ 21.0 kg/m2). Conclusion: We conclude that the detrimental effect of low prepregnancy BMI in Japanese women on birthweight and incidence of SGA infants. Our findings suggest that appropriate maternal BMI at conception followed by adequate weight gain during pregnancy may have a substantial influence on reducing the SGA infants and increasing the birthweight.
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