Percentage Change in Antenatal Body Mass Index as a Predictor of Neonatal Macrosomia

PURPOSE We wanted to evaluate the predictive value of percentage change in antenatal maternal body mass index (BMI) as it relates to macrosomia, as well as to compare change in pregnancy BMI with existing weight gain guidelines. METHODS We analyzed data from 6 months of consecutive deliveries, focusing on first visit (first trimester) BMI, last visit (37 weeks or later) BMI, and fetal birth weight. Using regression and chi(2) analyses, we evaluated the relationship between change in BMI and macrosomia. RESULTS Of the 238 consecutive deliveries, we were able to analyze data from 186, of which 15.6% (n = 29) of the infants were macrosomic. Among macrosomic infants, 86.2% (25/29) of their mothers had a 25% or greater increase in BMI compared with 6.6% (10/157) of mothers of normal-weight infants (P <.001), for a relative risk 13.5% (95% confidence interval [0], 7.3%-25.1%). Percentage change in BMI of 25% or greater had a sensitivity of 86.2% (95% Cl, 68.3%-96.1%), a specificity of 93.6% (95% Cl, 88.6%-96.9%), a positive predictive value of 71.4% (95% Cl, 53.7%-85.4%), and a negative predictive value 97.4% (95% Cl, 93.4%-99.3%) for macrosomia. Logistic regression adjusted for maternal age, race, parity, and gravidity showed that those women whose BMI increased 25% or greater were more than 200 times more likely (odds ratio [OR] = 219.3; 95% Cl, 38.8-1,238.6; P <.001) to give birth to a macrosomic infant. Further adjusting for initial BMI strengthened the association (OR = 1,062.4; 95% Cl, 83.2-13,572.2; P < 001). Regardless of weight gain, when compared with Institute of Medicine weight gain recommendations, change in BMI or 25% or greater was associated with macrosomia (P <.001). CONCLUSION Independent of initial pregnancy BMI or absolute weight gain, an increase in maternal BMI of 25% or greater during pregnancy is highly predictive of macrosomia. Ann Fam Med 2008:6;550-554. DOI: 10.1370/afm.903.