The Accuracy of Estimating Fetal Weight and Inter-Twin Weight Discordance by Ultrasound in Twin Pregnancies in Women With Increased Body Mass Index.

Objectives: The purpose of this study was to determine whether increased maternal pre-pregnancy BMI decreases the ultrasound accuracy of fetal weight estimation and inter-twin weight discordance in twin pregnancies compared with women with normal BMI.

Methods: We conducted a retrospective cohort study of women with a known pre-pregnancy or early pregnancy BMI who delivered a viable twin pregnancy after 28 weeks' gestation between 2008 and 2011, and who had an ultrasound examination for fetal weight estimation within two weeks of delivery. The sonographically estimated fetal weight (EFW) was compared with the actual weight for each twin, and inter-twin weight discordance (defined as a weight difference between twins of more than 25%) was stratified for the patient's BMI. We sought to determine if EFW and inter-twin weight discordance were affected if delivery occurred at eight to 14 days after ultrasound compared to within seven days of ultrasound.

Results: A total of 300 twin pregnancies with known pre-pregnancy maternal BMI were identified. Of these, 179 were underweight or of normal weight (BMI<25 kg/m2), 67 were overweight (BMI 25 to 29.9 kg/m2), and 54 were obese (BMI≥30 kg/m2). Ultrasound accuracy among all BMI groups were compared when done between 8 and 14 days and within seven days from delivery. There was a significant increasing trend in mean absolute percent errors for both twins in the obese compared to normal weight (P<0.05) if delivery happened between eight and 14 days from ultrasound. This difference was diminished if the ultrasound was performed within seven days of delivery. The ultrasound detection of inter-twin weight discordance was similar among the three BMI groups.

Conclusion: Estimation of fetal weight using ultrasound in obese women with twin pregnancies appears to be more reliable when performed close to delivery. Résumé.