Background: Discordant malformation between monochorionic twins is a rare and unknown phenomenon.Objectives: To estimate the incidence of discordant monochorionic twins and to describe their characteristics.Study design: A retrospective multicenter cohort of pregnancies between 2002 and 2015 in La Reunion Island was analyzed, thanks to a population-based register. Only monochorionic pregnancies were included in order to analyze specifically monozygotic twins. We defined as discordant twin pairs those in which different malformations were identified for each twin and those with only one fetus showing a malformation.Results: During the study period, 203,807 births occurred, including 410 monochorionic twin pairs. Congenital anomalies rate for monochorionic twin pairs was 10.7%. We included 38 monochorionic twin pairs with discordant phenotypes, which represent 9.3% of monochorionic twin pairs and 86.4% of monochorionic twin pairs affected by congenital anomalies. Among them, both twins were affected by different congenital anomalies in 7 pairs (18.4%), and only one twin was affected in 31 pairs (81.6%). We identified 20 congenital heart anomalies (44.4%), 5 brain anomalies (11.1%), 5 genital anomalies (11.1%), 4 axial bones and skull anomalies (8.9%), 4 limb anomalies (8.9%), 4 facial anomalies (8.9%), 3 urological anomalies (6.6%), 2 thoracic anomalies (4.4%), 1 bile duct anomaly (2,2%), 1 abdominal parietal defect (2.2%), and 1 aneuploidy (2.2%). Among them, 3 (6.6%) fetuses had an association of malformations. Among the 45 fetuses with malformations, 37 fetuses (82.2%) were born alive and 21 (46.6%) had postnatal surgery.Conclusions: Despite a supposed identical genome, discordant congenital anomalies in monochorionic twin pregnancies are not exceptional and related to genetic and epigenetic mechanisms. Sonographers and pediatricians should know that in monochorionic twin a pair, the occurrence of discordant phenotypes is high (9.3%).
Keywords: Congenital anomaly; fetus; monozygotic twins; registries; retrospective studies; twin pregnancy.