4 6 Increased monozygotic twinning rates have also been reported in selected pedigrees. No firm evidence exists for a genetic influence on monozygotic twinning, although interview studies of mothers of monozygotic twins report an excess of monozygotic twins among the mothers' relatives. 1 2 3 4 6 7 A study of genealogical records in Salt Lake City, USA, found a twinning rate of 1.45% among mothers who had a twin brother (dizygotic twins) and 1.65% for their sisters, compared with the rate of 0.69% for fathers who had a twin sister. 1 4 5 A genetic influence is indicated by the increased number of twin births among female relatives of women who had given birth to dizygotic twins. 1 2 3 4 The risk seems to exist only for dizygotic twinning. This has not yet been thoroughly investigated.Īmong women who have already given birth to twins the risk for further twin births is increased even after age effects are controlled for. It is generally agreed that genetic factors are important for dizygotic but not for monozygotic twinning.
Genetic components of monozygotic and dizygotic twinning seem to be independent. This is the result of an excess of monozygotic twins (39 pairs estimated, 18 expected).Ĭonclusion: Women who are twins have an increased risk of giving birth to twins.
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). 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. Main outcome measures: Numbers of monozygotic and dizygotic twin births expected and estimated. Subjects: Multiple births among 31586 deliveries between 19 to women who were twins. Objectives: To estimate the risk of having twin infants for mothers who are twins to investigate the genetic influence on twinning.ĭesign: Retrospective study of multiple births in two nationwide registries.
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