INTERTWIN DISPARITY IN FETAL SIZE IN MONOCHORIONIC AND DICHORIONIC PREGNANCIES

Objective: To compare monochorionic and dichorionic pregnancies for in tertwin disparities in fetal size. Methods: Monochorionic and dichorio nic pregnancies, recruited from an ultrasound screening study at 10-14 weeks' gestation, were compared for intertwin disparities in crown-ru mp length and birth weight. The disparities were expressed as a percen tage of the values of the larger twin. Results: The study population w as 123 monochorionic and 416 dichorionic twin pregnancies. In the 104 monochorionic and 381 dichorionic pregnancies resulting in two live bi rths, there were no significant differences in median (range) intertwi n disparity in crown-ramp length (4.3% [0-18.8%] and 3.4% [0-25.5%]) o r 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 birt h weight between the two groups were significant at alpha = .05 with 8 0% power, we would have had to examine a minimum of 984 and 926 twin p regnancies, respectively, assuming that the proportion of monochorioni c to dichorionic twins remained the same as in the current study. In a ddition, there was no significant correlation between intertwin dispar ities in crown-rump length and intertwin disparities in birth weight i n either the monochorionic (P = .40, Rho = 0.02, 95% confidence interv al [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 le ngth in 15 dichorionic pregnancies with chromosomally abnormal fetuses (6.6% [0-24.0%]) and in 20 dichorionic pregnancies that ended in misc arriage 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 pregn ancy outcome there was no significant difference in median (range) int ertwin disparity in crown-rump length (4.5% [0-20.0%]) from monochorio nic pregnancies resulting in two live births (4.3% [0-18.8%]). To dete rmine that the observed 0.2% difference in intertwin difference in cro wn-rump length between the two groups was significant at alpha = .05 w ith 80% power we would have had to examine a minimum of 5652 monochori onic twin pregancies, assuming that the proportion in each group remai ned the same as in the current study.Conclusion: The findings of this study demonstrate that monochorionic and dichorionic twin pregnancies do not differ significantly in intertwin disparity in fetal size, eith er in early pregnancy or at birth. (C) 1998 by The American College of Obstetricians and Gynecologists.