The concordance rate of non‐chromosomal congenital malformations in twins based on zygosity: a retrospective cohort study

Objective To examine the concordance rate of non‐chromosomal congenital malformations in twin pairs based on zygosity. Design Retrospective cohort study. Setting A tertiary hospital in Korea. Population Twin pairs born at Seoul National University Hospital between 2001 and 2019. Methods Congenital malformations were diagnosed by postnatal workups of neonates or autopsy in cases of stillborn infants. Zygosity was confirmed by sex, chorionicity and DNA analysis. Main outcome measures Concordance rate of congenital malformations in twin pairs based on zygosity. Results In total, 3386 twin pairs were included. The risk of a congenital malformation in the index twin increased significantly if the co‐twin had the congenital malformation, and the concordance rate was higher in monozygotic (MZ) than in dizygotic (DZ) twins (37.04 versus 16.77, P