The prevalence of sexual activity among never-married U.S. teenage women increased by 30 percent between 1971 and 1976; so that by age 19, 55 percent have had sexual intercourse. The increase, which has occurred at all ages and among all races, has been accompanied by a dramatic increase in the use of contraception, the use of the most effective methods, and the more regular use of all methods. Nevertheless, few teenagers begin use of contraception at the same time that they initiate intercourse--and many wait until after they have experienced pregnancy.
Zelnik et al. (1981) studied this question.