Weight gain during pregnancy and the risk of severe maternal morbidity by prepregnancy BMI

Background: High and low prepregnancy BMI are risk factors for severe maternal morbidity (SMM), but the contribution of gestational weight gain (GWG) is not well understood. Objectives: We evaluated associations between GWG and SMM by prepregnancy BMI group. Methods: We analyzed administrative records from 2.483,684 Californian births (2007-2012), utilizing z score charts to standardize GWG for gestational duration. We fit the z scores nonlinearly and categorized GWG as above, within, or below the Institute of Medicine (IOM) recommendations after predicting equivalent GWG at term from the z score charts. SMM was defined using a validated index. Associations were estimated using multivariable logistic regression models. Results: We found generally shallow U-shaped relations between GWG z score and SMM in all BMI groups, except class 3 obesity (>= 40 kg/m(2)), for which risk was lowest with weight loss. The weight gain amount associated with the lowest risk of SMM was within the IOM recommendations for underweight and class 2 obesity, but above the IOM recommendations for normal weight, overweight, and class 1 obesity. The adjusted risk ratios (RRs) and 95% CIs for GWG below the IOM recommendations, compared with GWG within the recommendations, were the following for underweight, normal weight. overweight, class 1 obesity, class 2 obesity, and class 3 obesity: 1.13 (0.99, 1.29), 1.09 (1.04, 1.14), 1.10 (1.01, 1.19), 1.07 (0.95. 1.21), 1.03 (0.88. 1.22), and 0.89 (0.73,1.08). respectively. For GWG above the recommendations, the corresponding RRs and 95% CIs were 0.99 (0.84, 1.15). 1.04 (0.99. 1.08), 0.98 (0.92, 1.04), 1.03 (0.95, 1.13), 1.07 (0.94, 1.23), and 1.08 (0.91, 1.30), respectively. Conclusions: High and low GWG may be modestly associated with increased risk of SMM across BMI groups, except in women with class 3 obesity, for whom low weight gain and weight loss may be associated with decreased risk of SMM.