Rates of small for gestational age and low birth weight among underweight and normal weight women.

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Prepregnancy underweight, and gestational weight gain has been associated with increased risk of adverse pregnancy outcomes, including preterm birth, low birthweight (LBW) and small for gestational age (SGA), but with conflicting results. The objectives were to compare the incidence of SGA, LBW, and other pregnancy outcomes between prepregnancy underweight and normal weight women and to evaluate possible associated risk factors. A retrospective cohort study was conducted in 220 underweight women (prepregnancy BMI of <18.5 kg/m2) and 440 normal weight women (prepregnancy BMI 18.5-24.9 kg/m2). Data were extracted from medical records and compared between the 2 groups, including baseline and obstetric characteristics, labor and delivery data, pregnancy, and neonatal outcomes. Underweight women were significantly younger and more likely to be nulliparous. They were significantly more likely to have weight gain below recommendation (33.6% vs. 23.2%, P<0.001). SGA and LBW were significantly more common in underweight compared to normal weight women (10.9% vs. 7%, P=0.034 and 13.2 vs. 7.3%, P=0.013, respectively). Other adverse neonatal outcomes were comparable. Logistic regression analysis showed that inadequate weight gain was the independent risk for both SGA and LBW (adjusted OR 2.20, 95%CI 1.19-4.09, P=0.012) and adjusted OR 2.31, 95%CI 1.28-4.159, P=0.005, respectively). Risk of both SGA and LBW were significantly increased in underweight compared to normal weight women. Inadequate weight gain was independently associated with increased risk of both SGA and LBW.