The Prospective Study of the Fragile X Syndrome is a large collaborative effort designed to collect prospective data on the pregnancy outcomes of individuals who carry the fragile X mutation. The goal of this 5-year study is to obtain empiric recurrence risks and population parameters for the fragile X syndrome in order to characterize the underlying mechanism of the mutation and the factors that influence its expression. This report presents the DNA results on the first 152 cases of female carriers and their pregnancy outcomes. It was found that the sex ratio of conceptuses was not significantly different from 1.0 and was not associated with mutation status. Thus, there was no evidence for selection against zygotes with full mutations. There was a significant association between the form of the mutation in carrier mothers and the frequency of its transmission. Examination of the segregation ratios from premutation mothers showed that there was a deficit of conceptuses that received the fragile X mutation. The segregation ratio from full mutation carrier mothers did not differ from expected. Several explanations for this observation are discussed. Numbers of cases are too small to estimate recurrence risks; however, the general trend of the data confirm the association of recurrence risks and the repeat number carried by the mother.