The National Institute of Child Health and Human Development (NICHD) created the NICHD Maternal Fetal Medicine Units Network in 1986 to conduct randomized trials and observational studies in perinatal medicine to improve adverse pregnancy and infant outcomes. From 1986 to 2002, the Network has started 16 randomized trials. Five of the trials are described, with particular attention given to difficult issues that arose, such as feasibility, sample size estimation, randomization in very high risk pregnancies, changing clinical practice, importance of the study question to the investigators, and lack of recruitment. Changes that the Network group made to their organization and methodology as a result of these issues are described, together with their application to some of the other trials proposed and conducted by the group.