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-R etrospective study of multiple births in two nationwide registries. Se tting-Sweden. Subjects-Multiple births among 31586 deliveries between 1973 and 1991 to women who were twins. Main outcome measures-Numbers o f monozygotic and dizygotic twin births expected and estimated. Result s-Women who are dizygotic twins have a moderately increased risk of ha ving 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 o f an excess of monozygotic twins (39 pairs estimated, 18 expected). Co nclusion-Women who are twins have an increased risk of giving birth to twins. Genetic components of monozygotic and dizygotic twinning seem to be independent.