Social differentials in cumulative fertility revealed by field surveys in a number of countries typically have been interpreted with the implicit assumption that the timing of childbearing within the reproductive period remains constant. However, the reproductive histories that have been collected in such surveys provide a largely unexploited source of data for the analysis of trends in timing. An analysis by birth cohort of the fertility experience of 592 ever-married women surveyed in San Juan, Puerto Rico, in 1966 shows that significant changes in timing have occurred. Women in the more recent birth cohorts are distinguished by earlier childbearing and sustained higher fertility in consecutive age periods. The inter-cohort differences remain when adjustments are made for duration of time in legal and consensual unions.