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Improved Ultrasonographic Estimation of Birth Weight in Macrosomic Fetuses by Application of a Correction Factor to the Gestation-Adjusted Projection Method

Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Madigan Army Medical Center, Tacoma, Washington USA.

Address correspondence to Craig M. Zelig, MD, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Madigan Army Medical Center, 9040A Fitzsimmons Ave, Tacoma, WA 98431 USA. E-mail: craig.zelig{at}amedd.army.mil

Objective. The purpose of this study was to improve estimated birth weight (EBW) determination in macrosomic fetuses (estimated fetal weight 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.

Key Words: birth weight • gestation-adjusted projection • ultrasonography

Abbreviations: BW, birth weight • EBW, estimated birth weight • EBW_CORR, corrected estimated birth weight • EBW_GAP, estimated birth weight by gestation-adjusted projection • EFW, estimated fetal weight • EFW_US, estimated fetal weight on ultrasonography • EGA, estimated gestational age • FWR, fetal weight ratio • GA, gestational age • GAP, gestation-adjusted projection • GL, gestational length • MFW_DEL, median fetal weight at delivery • MFW_US, median fetal weight on ultrasonography • PPV, positive predictive value • SEBW, signed error of the birth weight