Improved ultrasonographic estimation of birth weight in macrosomic fetuses by application of a correction factor to the gestation-adjusted projection method.

Objective: The purpose of this study was to improve estimated birth weight (EBW) determination in macrosomic fetuses (estimated fetal weight >or=4000 g) by application of a correction factor to the gestation-adjusted projection (GAP) method.

Methods: A review was performed of 411 singleton pregnancies delivered at term. On the basis of ultrasonographic examinations previously performed between 34.0 and 36.9 weeks' gestation, an EBW was calculated for each patient by the GAP method (EBW(GAP)). Using linear regression, a correction factor was developed that minimized the systematic error in the EBW(GAP). The model was then tested retrospectively on a second group of 317 patients.

Results: The GAP method systematically overestimated weights of the heavier fetuses in our population. The model we derived showed improved accuracy compared with the GAP method. When applied to a second group of 317 patients, our correction to the GAP method improved specificity for macrosomia from 94.7% to 98.6% (P = .003). Stated differently, the false-positive rate was reduced from 5.3% to 1.4%. The difference in sensitivity for macrosomia was not significant: 41.2% and 35.3% (P = .68).

Conclusions: Application of our model to our study population reduced the number of false-positive results for fetal macrosomia.