Vitamin D deficiency is common among otherwise healthy pregnant women and may have consequences for them as well as the early development and long-term health of their children. The importance of maternal and neonatal vitamin D status on later health has not been widely studied. The D-tect is a project financed by the Danish Council for Strategic research, comprising a battery of observational studies on associations between exposure to/status of vitamin D early in life and risk of various diseases later in life. All the studies use data from the Danish National registers. The fact that mandatory fortification of margarine with vitamin D in Denmark was cancelled in 1985, and thus, children born before and after 1985 differed by prenatal exposure to extra vitamin D, is utilized. Additionally, neonatal vitamin D status is measured using dried blood spots (PKU cards) kept in the The Danish Newborn Screening Biobank.