Discordance for Defects in Monochorionic Twins: Prevalence and Impact on Perinatal Outcomes

Background: Monozygotic twin pregnancies are at increased risk of congenital abnormalities compared to singletons. In 20% of cases, both fetuses are affected (concordance), while in 80% of cases, only one fetus is affected (discordance). This study examines the prevalence of discordance for structural defects in monochorionic (MC) twins, with normal aCGH comparative genomic hybridization (aCGH), reporting the types of detected abnormalities and their possible impact on perinatal outcomes, including the rate of single and double fetal loss before 24 weeks' gestation and the rate of preterm birth (PB) before 32 weeks' gestation. Methods: This was a retrospective study of discordant structural fetal anomalies in MC twin pregnancies detected at first-trimester scanning in three fetal medicine centers in Poland. Results: In the study population of 381 monochorionic twin pregnancies examined at 11-13 weeks' gestation, 21 (5.5%) pregnancies showed discordant structural defects with normal aCGH result. The most common were cardiac defects (n = 8), followed by central nervous system (CNS) (n = 6) defects and facial anomalies (n = 3). Single or double fetal loss before 28 weeks occurred in four (19%) and two (9%) cases, respectively, and was associated with intertwin crown-rump length (CRL) discordance greater than 20% (p = 0.046). PB before 32 weeks' gestation occurred in nine cases (47%) and was strongly associated with polyhydramnios (p = 0.001), which occurred mainly in CNS and facial defects. Conclusions: The prevalence of discordant structural defects with normal aCGH results among monochorionic twins is approximately 5%. In pregnancies with discordant defects, cardiac defects are the most common. Intertwin discordance greater than than 20% is associated with an increased risk of fetal demise.