BMJ 1996;312:879-881 (6 April)

Papers

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

Paul Lichtenstein, research associate,a Petra Otterblad Olausson, senior administrative officer,b A J Bengt Kallen, professor emeritus c

a Karolinska Institute, Institute for Environmental Medicine, Box 210, 17177 Stockholm, Sweden, b Centre for Epidemiology, National Board of Health and Welfare, 10630 Stockholm, Sweden, c Tornblad Institute, University of Lund, Biskopsg 7, 22362 Lund, Sweden

Correspondence to: Dr Lichtenstein.

Abstract

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


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