A prospective study of menstrual cycles and reproductive outcomes has been in progress since 1935. Data from this study are used to describe the risk of spontaneous abortion in two time periods 26 years apart Out of the total of 3889 women who have enrolled in this study, two cohorts of women are selected for analysis: cohort one consists of 2070 university students who entered the study in 1935–1944, and cohort two consists of 1375 students a generation later, 1961–1970. Cohort one has contributed 2408 pregnancies, and cohort two, 1493 pregnancies. Overall spontaneous abortion risk for the two cohorts is 16.9% and 13.1%, respectively. However age-specific risks of spontaneous abortion do not differ for the two groups, nor is there a difference in the gestational duration of spontaneously aborted pregnancies.