This book is the product of the efforts of a number of people dating back to 1956, when Professor T. Kemp, then head of the University Institute for Human Genetics in Copenhagen, proposed a study on the importance of x-ray irradiation in pregnant women. Under the guidance of Professors Dyhre Trolle and Preben Plum of the Uni versity Hospital in Copenhagen, the investigation was expanded to deal with prenatal and perinatal factors of importance for the development of the infant. The corpus of medical data that resulted from these efforts was collected and organized by Drs. Bengt Zachau-Christiansen and Aage Villumsen. The project's birth cohort included all deliveries that took place at the State University Hospital, Rigshospitalet, Copenhagen, over a two-year period from 1959 to 1961. As part of the perinatal study, the mothers and children were subjected to regular and thorough medical examinations during pregnancy and through the first year of the child's life. The detailed data from these examinations, as well as information about treatment administered, were systematically collected and coded.