Discordance for defects in monochorionic twins: prevalence and impact on perinatal outcomes

Abstract Background In monozygotic twin pregnancies the prevalence of congenital abnormalities is two times higher than in singletons. In 20% cases both fetuses are affected (concordance) but in 80% cases only on is affected (discordance). This study examines the prevalence of discordance for structural defects in monochorionic (MC) twins, report 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 before 32 weeks’ gestation. Methods Retrospective study for discordant structural fetal anomalies in monochorionic twins by the first trimester scan in three fetal medicine centers in Poland. Results In the study population of 381 monochorionic twins examined at 11–13 weeks’ gestation, there were 21 (5.5%) pregnancies with discordant structural defects and normal array comparative genomic hybridisation (aCGH) result. The most common were cardiac defects (n = 8), subsequently central nervous system (n = 6), facial (n = 3), abdominal wall (n = 3) and genitourinary system (n = 1) defects. Single fetal loss and double fetal loss before 28 weeks occurred in 4 (19%) and 2 (9%) cases respectively and was associated with intertwin discordance in crown-rump length (CRL) of more than 20% (p = 0.046). Preterm birth before 32 weeks’ gestation occurred in 9 cases (47%) and was strongly associated with polyhydramnios (p = 0.001) which occurred mainly in CNS and facial defects. Conclusions Monochorionic twins are at increased risk for structural abnormalities. In cases of discordant defects, the most common are cardiac defects. Intertwin discordance of more than 20%, rather than type of a defect, increases the risk single or double fetal demise.