The Danish Civil Registration System was established in 1968,25 where all people alive and living in Denmark were registered. It includes information on the unique personal identification number, sex, date and place of birth, parents’ identifiers and continuously updated information on emigration and death. The personal identification number is used in all national registers enabling accurate linkage within and between registers. The study base included all singleton births with known mothers born between 1 of May 1981 and 31 of December 2005, who were alive and resided in Denmark at their first birthday (N=1 472 762 persons). Selecting births in this period ensures individual samples to be retrieved in the Danish Neonatal Screening Biobank and reasonable distribution of cases and cohort members for all birth years. All residents are registered in the Danish Civil Registration System irrespective of health, income, receipt of social benefits, employment and other socioeconomic characteristics.26
