Infants of diabetic mothers have a two- to threefold increased risk of having congenital malformations. Primary prevention of these malformations may be possible if the teratogenic mechanisms in diabetes can be identified. The Diabetes in Early Pregnancy Study (DIEP) addresses the questions of high malformation rates and fetal losses using an innovative study design. Women are recruited prior to pregnancy, monitored to ensure prompt diagnosis of pregnancy, and followed carefully for diabetic problems or fetal losses. To date, 450 diabetic and 550 normal control prepregnant women have been recruited. A basal body temperature monitoring system has been successful in identifying pregnant women within 21 days of conception. Before initiating such a study, problems of feasibility, study design, and possible bias had to be solved. This report summarizes our experience in designing and conducting a prospective pregnancy study. The DIEP demonstrates that a truly prospective pregnancy study is feasible.