Effect of prepregnancy body mass index categories on obstetrical and neonatal outcomes

To examine the association between body mass index (BMI) and obstetrical and neonatal outcomes.

We conducted a cohort study comparing prepregnant BMI categories with obstetrical and neonatal outcomes using the McGill Obstetrical and Neonatal Database on all deliveries in 10 year period (1987–1997). Prepregnant BMI was categorized into underweight (<20), normal (20–24.9), overweight (25–29.9), obese (30–39.9), and morbidly obese (40+). Logistic regression analysis was used to adjust for age, smoking, parity, and preexisting diabetes using normal BMI as the reference.

The population consisted of underweight 4,312 (23.1%), normal weight 10,021 (53.8%), overweight 3,069 (16.5%), obese 1,137 (6.1%), and morbidly obese 104 (0.6%). As compared to women with normal BMIs, overweight, obese, and morbidly obese women had an increased risk of preeclampsia 2.28 (1.88–2.77), 4.65 (3.71–5.83), 6.26 (3.48–11.26); gestational hypertension 1.56 (1.35–1.81), 2.01 (1.64–2.45), 2.77 (1.60–4.78); gestational diabetes 1.89 (1.63–2.19), 3.22 (2.68–3.87), 4.71 (2.89–7.67); preterm birth 1.20 (1.04–1.38), 1.60 (1.32–1.94), 2.43 (1.46–4.05); cesarean section 1.48 (1.35–1.62), 1.85 (1.62–2.11), 2.92 (1.97–4.34); and macrosomia 1.66 (1.23–2.24), 2.32 (1.58–3.41), 2.10 (0.64–6.86). Underweight women were less likely to have: preeclampsia 0.67 (0.52–0.86), gestational hypertension 0.71 (0.60–0.83), gestational diabetes 0.82 (0.69–0.97), cesarean section 0.89 (0.81–0.97), shoulder dystocia 0.88 (0.80–0.96), birth injuries 0.40 (0.21–0.77), and macrosomia 0.43 (0.28–0.68) but more likely to have small for gestational age infants 1.54 (1.37–1.72) and intrauterine growth restricted infants 1.33 (1.07–1.67).

In a large Canadian teaching hospital, increasing prepregnancy BMI category was associated with an increasing risk of adverse pregnancy outcomes. Underweight prepregnancy BMI was associated with a reduced risk of adverse pregnancy outcomes.