Is intertwin birth weight discordance predictable?

Objective: To compare the efficacies of intertwin difference in abdominal circumferences (ACs) and estimated fetal weights (EFWs) to predict birth weight discordance in twin gestations. Methods: Ninety pairs of twins with sonographic measurements of AC and femur length, performed within 2 weeks before delivery, were included in the study. The EFW was calculated (Hadlock's formula). We used Bayes' theorem and the Mantel-Haenszel chi(2) test to calculate and compare the rates of false-positive and false-negative prediction of the cutoff values of 15, 20, and 25% intertwin birth weight discordance by the intertwin EFW difference and an intertwin AC difference of greater than or equal to 18 mm. Results: There was no significant difference between the two methods to predict birth weight discordance at 15-25% discordance (p = 0.254, OR = 0.33, 95% CI = 1.45-0.24) and at >25% discordance (p = 0.112, OR = 0.18, 95% CI = 0.03-1.34). Conclusions: Both the intertwin AC difference (greater than or equal to 18 mm) and intertwin EFW difference have similar negative predictive values and the same efficacy to exclude discordant growth. Because of the relatively low positive predictive values, birth weight discordance cannot be accurately predicted by either method.