Intertwin disparity in fetal size in monochorionic and dichorionic pregnancies

To compare monochorionic and dichorionic pregnancies for intertwin disparities in fetal size.

Monochorionic and dichorionic pregnancies, recruited from an ultrasound screening study at 10-14 weeks' gestation, were compared for intertwin disparities in crown-rump length and birth weight. The disparities were expressed as a percentage of the values of the larger twin.

The study population was 123 monochorionic and 416 dichorionic twin pregnancies. In the 104 monochorionic and 381 dichorionic pregnancies resulting in two live births, there were no significant differences in median (range) intertwin disparity in crown-rump length (4.3% [0-18.8%] and 3.4% [0-25.5%]) or birth weight (10.2% [0-37.0%] and 9.3% [0-49.2%]). To determine that the observed 0.9% intertwin differences in crown-rump length and birth weight between the two groups were significant at alpha = .05 with 80% power, we would have had to examine a minimum of 984 and 926 twin pregnancies, respectively, assuming that the proportion of monochorionic to dichorionic twins remained the same as in the current study. In addition, there was no significant correlation between intertwin disparities in crown-rump length and intertwin disparities in birth weight in either the monochorionic (P = .40, Rho = 0.02, 95% confidence interval [CI] -0.17, 0.22) or dichorionic group (P = .44, Rho = 0.01, 95% CI -0.11, 0.09). The median (range) intertwin disparity in crown-rump length in 15 dichorionic pregnancies with chromosomally abnormal fetuses (6.6% [0-24.0%]) and in 20 dichorionic pregnancies that ended in miscarriage or intrauterine death of one or both fetuses (7.7% [0-43.9%]) was significantly higher than in dichorionic pregnancies resulting in two live births (Z = 2.49 and 3.26, respectively, and P = .01 and .001, respectively). However, in 19 monochorionic twins with adverse pregnancy outcome there was no significant difference in median (range) intertwin disparity in crown-rump length (4.5% [0-20.0%]) from monochorionic pregnancies resulting in two live births (4.3% [0-18.8%]). To determine that the observed 0.2% difference in intertwin difference in crown-rump length between the two groups was significant at alpha = .05 with 80% power we would have had to examine a minimum of 5652 monochorionic twin pregnancies, assuming that the proportion in each group remained the same as in the current study.

The findings of this study demonstrate that monochorionic and dichorionic twin pregnancies do not differ significantly in intertwin disparity in fetal size, either in early pregnancy or at birth.