Fetal Weight Estimation in Diabetic Pregnancies Using the Gestation‐Adjusted Projection Method

Objectives The gestation‐adjusted projection method extrapolates birth weight using third‐trimester sonography. This technique is shown to be more accurate for sonographic examinations from 34 weeks to 36 weeks 6 days than 37 weeks to 38 weeks 6 days. Our objective was to determine whether even earlier sonographic examinations (31 weeks–33 weeks 6 days) further improves birth weight prediction in patients with diabetes. Methods We conducted a retrospective cohort analysis of 388 pregnant women with pregestational or gestational diabetes who delivered at 37 weeks or later and had a sonographic examination performed between 31 weeks and 36 weeks 6 days. Sonographic examinations were categorized as “early” if performed at 31 weeks to 33 weeks 6 days or “late” if performed at 34 weeks to 36 weeks 6 days. We estimated birth weight using the gestation‐adjusted projection method, compared errors in prediction of birth weight using the t test and Mann‐Whitney U test, and performed a 2‐sample test of proportions to compare prediction of macrosomia (birth weight >4000 g). Results The early and late groups had similar mean gestational ages at birth (38 weeks 4 days versus 38 weeks 5 days; P = .13) and rates of macrosomia (10.7% versus 12.4%; P = .63). The early group had a greater mean absolute error (336 versus 297 g; P = .03) and percent error (9.9% versus 7.9%; P = .01) in birth weight prediction but a lower mean birth weight (3303 versus 3426 g; P = .02). Sensitivity for prediction of macrosomia was 19% in the early group versus 45% in the late group ( P = .07), whereas specificity was similar (98% versus 96%; P = .27). Conclusions Using the gestation‐adjusted projection method in our patients with diabetes, we found that sonographic examinations performed at 34 weeks to 36 weeks 6 days better predicted birth weight than those performed at 31 weeks to 33 weeks 6 days.