Diagnosis of macrosomia in the postdates population: combining sonographic estimates of fetal weight with glucose challenge testing

OBJECTIVE: To evaluate the test characteristics of sonographic estimation of fetal weight in the detection of macrosomia in nondiabetic postdates patients as a function of maternal glucose value measured after glucose challenge testing performed at 24-28 weeks of gestation. METHODS: At or beyond 41 weeks' gestation, 656 nondiabetic patients had sonographic estimation of fetal weight. Receiver-operator characteristic curve analysis was used to define the glucose value at which an optimal number of macrosomic fetuses could be identified. The test characteristics of sonography in the prediction of macrosomia in the two populations defined by that cut-off value were evaluated. RESULTS: A glucose level of 120 mg/dL (6.6 mM) was identified as the optimal cutoff for prediction of birth weight > or = 4,000 g. In the group with a glucose level > or = 120 mg/dL, sonographic estimation of fetal weight in the detection of macrosomia offered a sensitivity, specificity, and positive and negative predictive values of 63%, 91%, 71%, and 86%, respectively. In those with glucose level <120 mg/dL, sonography demonstrated a sensitivity, specificity, and positive and negative predictive values of 65%, 89%, 60%, and 91%, respectively. CONCLUSIONS: In postdates nondiabetic patients, routine glucose challenge testing performed early in pregnancy has limited ability to improve the test characteristics of sonography to predict macrosomia. The positive predictive value of sonographically suspected macrosomia increases from 60-71% in patients whose glucose level was > or = 120 mg/dL (P = 0.002).