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incidence of adolescent pregnancy.3 Access to contraceptives has become increasingly crucial for adolescents because many are sexually active at earlier ages than in the past. During the mid-to-late 1950s, 8% of adolescent females had had intercourse by age 16. In contrast, by the early-to-mid–1980s, 21% of female teenagers had had sex by age 16. Among all adolescents in the 1990s, 50% had sex by age 18, compared with 27% of adolescents of similar ages in the 1950s. The proportions who had done so by age 20 were 76% and 61%, respectively.4 The delay in age at first marriage is another important trend with implications for unintended pregnancy. Between 1970 and 1994, the median age at first marriage among females rose from 20.8 to 24.5.5 The length of time between first intercourse and first marriage is estimated to be about seven years for women.6 Delayed first marriage, combined with earlier ages at first intercourse, results in an extended period of time during which women are at risk of nonmarital pregnancy. Although increased numbers of adolescents are sexually active, pregnancy rates among sexually experienced teenagers declined by 19% between 1972 and James G. Kahn is associate professor with the Department of Epidemiology and Biostatistics and Institute for Health Policy Studies, and Claire D. Brindis is associate professor with the Department of Pediatrics, Division of Adolescent Medicine and Institute for Health Policy Studies, both at University of California, San Francisco. Dana A. Glei is a graduate student in the Office of Population Research and the Department of Sociology, Princeton University, Princeton, NJ. The research on which this article is based was supported by The California Wellness Foundation. Pregnancies Averted Among U.S. Teenagers By the Use of Contraceptives