title: The new Danish Twin Register: establishment and analysis of twinning rates.
abstract: BACKGROUND
  Twin registers provide a valuable source for research into disease causation. The existing population-based registers comprise mostly old twins. In order to be able to study diseases which occur in childhood and youth a new Danish twin register has been established.
  
  
  METHODS
  The register is based on the Danish Civil Registration, with information on number of twin births from the Danish Vital Statistics Office as the source of validation. All twins resident in Denmark at 1 March 1991 were sent a one-side questionnaire asking about diabetes, willingness to participate in other research projects and similarity in the twins.
  
  
  RESULTS
  The register, comprising 20,888 twin pairs, covers 74.4% of all twin pairs born 1953-1967 (incl.) and 97.4% of those born 1968-1982 (incl.). The response rate to the questionnaire study was 92.3%. The responders represented 19,180 twin pairs distributed as 5304 monozygotic pairs, 6861 same-sex dizygotic pairs, 6244 opposite-sex dizygotic pairs and 771 pairs of unknown zygosity. Of the respondent twins, 96% declared their willingness to participate in additional studies. An analysis of trends in the twinning rates for the years 1968-1982 showed that the rate of monozygotic twinning is increasing and the twinning rate of opposite-sex twin pairs is decreasing.
  
  
  CONCLUSIONS
  Earlier estimated trends in twinning rates have been confirmed. Due to the high response rate and opportunities for linkage with other Danish registers, the present material provides a valuable resource for twin studies in diseases and human traits.
authors.affiliation: Genetic Epidemiology Research Unit, University of Odense, Denmark.
authors.forename: K O
authors.initials: KO
authors.lastname: Kyvik
authors.forename: A
authors.initials: A
authors.lastname: Green
authors.forename: H
authors.initials: H
authors.lastname: Beck-Nielsen
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D003718:Denmark; D003922:Diabetes Mellitus, Type 1 / Q000235:genetics; D005260:Female; D006801:Humans; D008297:Male; D012042:Registries; D011795:Surveys and Questionnaires; D014427:Twins / Q000235:genetics / Q000523:psychology / Q000706:statistics & numerical data*