This review discusses the use of Nordic Medical Birth Registers (MBRs) in epidemiological studies, paying special attention to the topics that have been investigated and to other data sources that increase the usefulness of MBRs. We divided the reviewed studies into four groups according to the data sources on which they were based: (1) studies using MBR data only; (2) studies combining maternal or paternal background information, obtained from other data sources, with MBR data; (3) studies combining MBR data with subsequent outcome information on newborns; and (4) studies using information about consecutive pregnancies and generations. Our review shows that MBRs are good sources for studying the following topics: maternal biological and obstetric background; interventions and health care during pregnancy and birth; newborns' short-term outcome; and the relations between these factors. In addition, the usefulness of MBRs increases when the data they contain are combined with data from other sources. We found that data from more than twenty different sources have been linked with MBR data in the Nordic counties. As time passes, national MBRs become a useful source of information for studies on consecutive pregnancies or generations. In addition, the associations between pregnancy, delivery, perinatal health and long-term outcome can be studied by using an MBR as the basic data source.