More than 40% of all births in the United States occur to unmarried women, and nonmarital birth rates are higher among women in their early twenties than among women in any other age group.  Although it has declined somewhat in recent years, the birth rate among unmarried women aged 20-24 grew by about 25% over the past quarter century.  A substantial percentage of nonmarital pregnancies (both within and outside of cohabitation) are unintended, and nonmarital childbearing has been found to be associated with reductions in educational attainment and labor force participation among mothers and with increases in the incidence of poverty and academic problems among children.  A number of well-designed studies have found that these relationships are at least in part causal.  This paper reports findings from a series of simulations measuring the extent to which contraceptive use and/or contraceptive efficacy among unmarried women would have to increase in order to achieve a 25% reduction in nonmarital birth rates among women in their early twenties.  All simulations were performed using the Brookings Institution's FamilyScape 2.0 microsimulation model of family formation, which was developed by researchers at Brookings, Child Trends, and Georgetown University.  Preliminary results suggest that the targeted reduction in nonmarital birth rates could be achieved by lowering the current proportion of young men and women who do not use contraception.  Initial findings also indicate that it may be more difficult to realize such a large reduction in nonmarital births via improvements in the consistency and effectiveness of current method use or changes in method mix among existing contraceptors (by shifting users from methods such as oral contraception to longer acting methods such as injectables, IUDs or implants) unless such changes are accompanied by reductions in non-contraception.