Effect of Prepregnancy Body Mass Index Categories on Obstetrical and Neonatal Outcomes

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

Methods: 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.

Results: 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).

Conclusion: 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.