After 41 consecutive years of increase, out-of-wedlock teen childbearing unexpectedly reversed course in 1991 and by 2002 was 20% below its peak. Explanations for that reversal have proven elusive. In this paper, we present evidence that exposure to legalized abortion in utero for the cohort of women that became teenagers in the 1990s is one factor contributing to this decline. We estimate that the legalization of abortion in the 1970s changed the composition of women at risk of bearing children out of wedlock some 15-24 years later. This composition effect reduced out-of-wedlock teen birth rates by 6%, which accounts for roughly 25% of the observed decline in unmarried teen birth rates over this period. It also lowered rates of unmarried births for women aged 20-24. At the same time, it increased the number of married births to women 20-24, so that there is only a small reduction in total fertility over the ages 15-24. The detailed information available on birth certificates enables a more direct identification of in utero abortion exposure than prior studies looking at other outcomes such as crime.