Twins born between 1953 and 1982 were ascertained via CRS, too, although another method was used, as the linkage between family members is more reliable when it comes to these cohorts. CRS ascertained all women who during this period had born more than one child within three days. The latter to allow for the fact that twins can be born on either side of midnight – however, this only turned out to be the case in less than 0.5% of all twin deliveries during these years. All these children were then recorded in The Danish Twin Registry. This part of the Registry includes about 20,000 twin pairs, and in 1991 a questionnaire was sent to all of those who were alive and who were living in Denmark at the time (about 39,000 twins).