Estimators of birth weight in pregnant women requiring insulin: a comparison of seven sonographic models.

OBJECTIVE: To determine if the relative accuracy of the sonographic estimate of birth weight among diabetic gravidas requiring insulin improves significantly as more fetal measurements are used.
METHODS: We studied 172 diabetic women requiring insulin who had sonographic measurements of fetal parts within 7 days of delivery. Friedman nonparametric analysis of variance followed by Dunn multiple comparison and chi 2 were used to assess the relative accuracy of the seven models. Prediction limits were calculated to determine the estimate of fetal weight that would ensure (with 90% accuracy) that the newborn was macrosomic (at least 4 kg).
RESULTS: The mean (+/- standard deviation [SD]) birth weight was 3388 +/- 727 g, and the frequency of macrosomia at term gestation was 19.4% (29 of 149). The mean standardized absolute error (g/kg) based on abdominal circumference (AC) and femur length (FL) (86 +/- 72 g/kg) was not significantly different from the other models (range 84 +/- 72 to 116 +/- 99 g/kg, P > .05). The percent of estimate within 10% of actual birth weight using AC and FL (65%) was similar to the other models (53.4-66.2%). Regardless of the White classification, the estimation of fetal weight using AC and FL had an accuracy similar to the other six regression equations. To ensure that the birth weight is at most 3999 g, the estimated fetal weight should be 3200 g, and, conversely, if the predicted weight is 4700 g, then the newborn is macrosomic.
CONCLUSION: Among patients requiring insulin, estimation of birth weight using AC and FL is as accurate as more complicated models. The ability to detect macrosomia by ultrasound is limited.