THE NEW DANISH TWIN REGISTER - ESTABLISHMENT AND ANALYSIS OF TWINNINGRATES

Background. Twin registers provide a valuable source for research into disease causation. The existing population-based registers comprise m ostly old twins. In order to be able to study diseases which occur in childhood and youth a new Danish twin register has been established. M ethods. The register is based on the Danish Civil Registration, with i nformation on number of twin births from the Danish Vital Statistics O ffice as the source of validation. All twins resident in Denmark at 1 March 1991 were sent a one-side questionnaire asking about diabetes, w illingness to participate in other research projects and similarity in the twins. Results. The register, comprising 20 888 twin pairs, cover s 74.4% of all twin pairs born 1953-1967 (incl.) and 97.4% of those bo rn 1968-1982 (incl.). The response rate to the questionnaire study was 92.3%. The responders represented 19 180 twin pairs distributed as 53 04 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 stu dies. An analysis of trends in the twinning rates for the years 1968-1 982 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 Danis h registers, the present material provides a valuable resource for twi n studies in diseases and human traits.