Correctly identifying the macrosomic fetus: Improving ultrasonography-based prediction

OBJECTIVE: Our goal was to improve the accuracy of estimating fetal weights among macrosomic fetuses with the traditional measurements of abdominal ci rcumference, femur length, and head circumference. STUDY DESIGN: We used 48 31 cases without anomalies from an ultrasonography laboratory database with an estimated fetal weight obtained a maximum of 14 days before delivery. A bdominal circumference, femur length, and head circumference were each regr essed on birth weight to obtain estimated fetal weight by abdominal circumf erence. femur length, and head circumference, respectively. We compared the individual variation for estimated fetal weight by abdominal circumference , femur length, and head circumference by calculating a within-subject stan dard deviation to quantify the level of disparity. We adjusted the estimate d Petal weight to the date of delivery and for dependencies on maternal dia betes mellitus, weight, and height. We then weighted cases with birth weigh t >4500 g and diabetic cases with birth weight >4000 g 20-fold (weighted es timated fetal weight) for the purpose of creating a favorable bias for clas sifying these cases. The equation of Hadlock et at, with abdominal circumfe rence, femur length, and head circumference, was applied as a benchmark est imated fetal weight. RESULTS: Of the 4831 newborns, 308 (6.4%) had a birth weight >4000 g, and 5 6 (1.2%) had a birth weight >4500 g. There were 154 pregnancies complicated by diabetes mellitus; 26 (16.9%) of the resulting infants weighed >4000 g, and 5 (3.2%) weighed >4500 g. At 95% specificity the weighted estimated fe tal weight had a sensitivity of 85.7% at a cut point of 3912 g, compared wi th a sensitivity of 71.4% at 3604 g by use of the estimated fetal weight of Hadlock et al. CONCLUSIONS: We were able to improve the accuracy of identifying the macros omic fetus compared to reliance on the equation by Hadlock et at. A fetus w as found to be at significantly increased risk for birth weight >4000 g whe n the estimated fetal weight based on abdominal circumference is larger tha n that based on either head circumference or femur length or when there is a large within-subject variance in estimated fetal weight based on abdomina l circumference, femur length, and head circumference. We also found that t here were significantly different groups of patients whose estimated fetal weights require different equations for better estimates. Even given ultras onographic measurements, taking into account maternal height, weight, and p resence of diabetes mellitus can improve macrosomia detection. Although the se findings remain to be optimized and validated, the approach used here ap pears to yield better predictions than the current "one function fits all" approach.