To determine whether fetal ultrasound early in the third trimester can identify Latina with mild gestational diabetes mellitus (GDM) whose fetuses are at risk for macrosomia and, if so, whether maternal insulin therapy can reduce that risk.

Study subjects included 303 consecutive women with GDM and a fasting serum glucose level <5.8 mM on diet therapy who had a fetal ultrasound between 29 and 33 weeks gestation. Of the women, 98 (32%) had a fetal AC ≥ 75^th percentile for gestational age, and 59 women completed a randomized trial of diet therapy (n = 29) or diet plus twice daily insulin (n = 30). Maternal nutrient levels were assessed by meal tolerance testing (MTT) before and during therapy and by capillary glucose monitoring four to seven times a day. Birth weights corrected for gestational age and neonatal glycemia and skin folds were the primary outcome variables compared between treatment groups.

Diet and diet-plus-insulin groups were well matched for maternal age, prepregnancy relative weight, weight gain during pregnancy, and glycemia at entry. Insulin therapy reduced maternal capillary (P < 0.005) and MTT (P < 0.001) glucose levels and prevented a diet-associated rise in MTT triglyceride levels (P < 0.002). Gestational age at delivery was similar in insulin- and diet-treated groups (39.6 ± 0.2 vs. 39.5 ± 0.2 weeks). Birth weights (3,647 ± 67 vs. 3,878 ± 84 g; P < 0.02), the prevalence of large-for-gestational age infants (13 vs. 45%, P < 0.02), and neonatal skin-fold measurements at three sites (P < 0.005) were reduced in the insulin-treated group. Rates of transient neonatal hypoglycemia were low in both treatment groups (14 and 18%, respectively) and didnot differ significantly between groups.

Fetal ultrasound early in the third trimester identified women with mild GDM whose infants were at high risk for fetal macrosomia in the absence of standard glycemic criteria for insulin therapy. Insulin treatment reduced the macrosomia, indicating that fetal ultrasound can be used to guide metabolic therapy in pregnancies complicated by mild GDM.