The study population.

A collaborative perinatal study was conducted during the January 1959-December 1965 period. 15 university-affiliated medical centers were selected for participation in the study; these are listed in this chapter. Tables present the distribution of data on the following factors for the sampling frame and the ultimate sample for each of the participating institutions: 1) marital status; 2) age of gravida; 3) gestation at registration; and 4) ethnic group. Differences in the ways each institution selected its sampling are discussed. The characteristics of the women lost to further study are mentioned. A discussion is devoted to the way in which the sample was maintained over the 1st year.