Prenatal determination of chorionicity in triplet pregnancy by ultrasonographic examination of the ipsilon zone

Objective: To determine chorionicity in triplet pregnancies by ultrasonographic assessment of the ipsilon zone, the junction of the three interfetal membranes. Methods: The thickness of the component membranes in the ipsilon zone was studied to determine chorionicity in 28 triplet pregnancies, by retrospective examination of the ultrasonographic images taken at 9-24 weeks' gestation, and in 20 consecutive triplet pregnancies followed prospectively by targeted ultrasonography at 8-21 weeks' gestation. Prenatal ultrasonographic findings were compared with those obtained from the records of the infertility centers or referring hospitals (the number of gestational sacs and live embryos in each sac seen by transvaginal scanning at 6-7 weeks' gestation). Results: Of the 28 triplet pregnancies with appropriate images demonstrating the ipsilon zone, 22 were classified as trichorionic, five dichorionic, and one monochorionic. This classification was correct in all but one trichorionic pregnancy, which was misclassified as dichorionic. In the prospective subset (n = 20) there were 16 trichorionic and four dichorionic triplet pregnancies. The ipsilon zone was not present in one case in which the interfetal membrane did not intersect. In the remaining 19 pregnancies, there was a complete correlation between the findings at the ipsilon zone and transvaginal ultrasonography at 6-7 weeks. Conclusion: Ultrasonographic assessment of the ipsilon zone is useful for predicting chorionicity in triplet pregnancies.