Objective: To determine the incidence of structural heart disease in at least one of a monochorionic (MC) twin pair excluding any cardiac effects of twin-twin transfusion syndrome (TTTS).
Method: A cohort study of 165 sets of MC twins undergoing detailed fetal echocardiography in a tertiary unit over a 4-year period.
Results: The overall risk of at least one of a MC twin pair having a structural congenital cardiac anomaly was 9.1% (15/165); for monochorionic diamniotic (MC/DA) twins, this figure was 7.0% (11/158) but for monochorionic-monoamniotic (MC/MA) twins the risk for at least one affected twin was 57.1% (4/7). If one of a pair of MC twins was affected, the risk to the other twin for a structural cardiac anomaly was 26.7% (4/15).
Conclusions: The incidence of structural heart disease in MC twins is increased, independent of TTTS. This justifies referral of these pregnancies for detailed fetal echocardiography as part of their assessment. If one twin is affected, the risk to the other twin is increased further.