Trends in ages at key reproductive transitions in the United States, 1951-2010.

BACKGROUND: Key sexual and reproductive health milestones typically mark changing life stages with different fertility intentions and family planning needs. Knowing the typical ages at such events contributes to our understanding of changes in family formation and transition to adulthood and helps inform needs for reproductive health services.
METHODS: We used data from the 1982-2010 National Surveys of Family Growth and the 1995 National Survey of Adolescent Males and event history methods to examine trends over time for women and men in the median ages at several reproductive and demographic events.
FINDINGS: Women's reports indicate that age at menarche has changed little since 1951. Women's and men's median ages at first sex declined through the 1978 birth cohort, but increased slightly since then, to 17.8 years for women and 18.1 for men. The interval from first sex to first contraceptive use has narrowed, although Hispanic women have a longer interval. Age at first union (defined as the earlier of first marriage or first cohabiting relationship) has remained relatively stable, but the time between median age at first sex and median age at first birth has increased to 9.2 years for women and 11.4 for men. For some women and men born in the late 1970s, median age at first birth was earlier than median age at first marriage for the first time in at least the past several decades.
CONCLUSION: The large majority of the reproductive years are spent sexually active. Thus, women have a lengthy period during which they require effective methods. In particular, the period between first sex and first childbearing has lengthened, but long-acting method use, although increasing, has not kept up with this shift. Moving the contraceptive method mix toward underutilized but highly effective contraceptive methods has the potential to reduce the unintended pregnancy rate.