title: Twin births to mothers who are twins: a registry based study.
abstract: OBJECTIVE
  To estimate the risk of having twin infants for mothers who are twins; to investigate the genetic influence on twinning.
  
  
  METHODS
  Retrospective study of multiple births in two nationwide registries.
  
  
  METHODS
  Sweden.
  
  
  METHODS
  Multiple births among 31,586 deliveries between 1973 and 1991 to women who were twins.
  
  
  METHODS
  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.
authors.affiliation: Karolinska Institute, Institute for Environmental Medicine, Stockholm, Sweden
authors.forename: P
authors.initials: P
authors.lastname: Lichtenstein
authors.forename: P O
authors.initials: PO
authors.lastname: Olausson
authors.forename: A J
authors.initials: AJ
authors.lastname: Källén
mesh_terms: D005260:Female; D006801:Humans; D009035:Mothers; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012042:Registries; D012307:Risk Factors; D012723:Sex; D013548:Sweden / Q000453:epidemiology; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic