The Copenhagen Perinatal Cohort comprises 9125 individuals delivered by 8949 women from October 1959 to December 1961 at the Copenhagen University Hospital, Rigshospitalet. The mothers were mainly residents in Copenhagen, but some non-residents were admitted on obstetrical indications (Zachau-Christiansen & Ross, 1975). A total of 8400 infants survived the first month after birth. Personal identification numbers, which are required to obtain information from Danish health registers, were available for 7459 parent-child pairs. From this sample second born twins and siblings were excluded, resulting in a study sample comprising 7177 cohort members (3627 men and 3550 women) and their parents. The mean age of the mothers when they gave birth was 26 years and the mean age of the fathers was 30 years.
