This book discusses the problems in studying the effect of maternal drug use during pregnancy and delivery outcome, including congenital malformations and other defects of the infants. For each main category of relevant drugs, information gathered from the literature and data obtained from the Swedish Medical Birth Register are used. This register is a rather unique source of information because during the time interval studies, nearly one million women have been interviewed in early pregnancy about the use of drugs during pregnancy and data from their deliveries and their infants have been collected. This book will contribute to the scientific literature because of much of the data have never been published as scientific articles and previously published data will be updated