Fetal weight estimation in diabetic pregnancies using the gestation-adjusted projection method: comparison of two timing strategies for third-trimester sonography.

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.