Twin births to mothers who are twins: a registry based study

Objectives : To estimate the risk of having twin infants for mothers who are twins; to investigate the genetic influence on twinning. Design : Retrospective study of multiple births in two nationwide registries. Setting : Sweden. Subjects : Multiple births among 31586 deliveries between 1973 and 1991 to women who were twins. Main outcome measures : Numbers of monozygotic and dizygotic twin births expected and estimated. Results : Women who are dizygotic twins have a moderately increased risk of having twins (relative risk 1.30, 95% confidence interval 1.14 to 1.49) which seems to be completely the result of dizygotic twinning. When a mother is a monozygotic twin, her risk of having twins of the same sex is significantly increased (1.47; 1.10 to 1.97). This is the result of an excess of monozygotic twins (39 pairs estimated, 18 expected). Conclusion : Women who are twins have an increased risk of giving birth to twins. Genetic components of monozygotic and dizygotic twinning seem to be independent. Key messages It is generally agreed that genetic factors are important for dizygotic but not for monozygotic twinning Women who themselves are dizygotic twins have an increased risk of giving birth to dizygotic twins Women who themselves are monozygotic twins have an increased risk of having monozygotic twins The genetic components in monozygotic and dizygotic twinning are independent