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Twin births to mothers who are twins: a registry based study
P Lichtenstein1, P O Olausson, A J Källén
1Karolinska Institute, Institute for Environmental Medicine, Stockholm, Sweden
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 31,586 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).
Conclusions:
Women who are twins have an increased risk of giving birth to twins. Genetic components of monozygotic and dizygotic twinning seem to be independent.