The Danish Medical Birth Register was established in 1973. It is a key component of the Danish health information system. The register enables monitoring of the health of pregnant women and their offspring, it provides data for quality assessment of the perinatal care in Denmark, and it is used extensively for research. The register underwent major changes in construction and content in 1997, and new variables have been added during the last 20 years. The aim was to provide an updated description of the register focusing on structure, content, and coverage since 1997. The register includes data on all births in Denmark and comprises primarily of data from the Danish National Patient Registry supplemented with forms on home deliveries and stillbirths. It contains information on maternal age provided by the Civil Registration System. Information on pre-pregnancy body mass index and smoking in first trimester is collected in early pregnancy (first antenatal visit). The individual-level data can be linked to other Danish health registers such as the National Patient Registry and the Danish National Prescription Registry. The register informs several other registers/databases such as the Danish Twin Registry and the Danish Fetal Medicine Database. Aggregated data can be publicly accessed on the Danish Health Data Authority web page ( www.esundhed.dk/sundhedsregistre/MFR ). Researchers can obtain access to individual-level pseudo-anonymised data via servers at Statistics Denmark and the Danish Health Data Authority.