This paper examines the relationship between migration and premarital childbearing in a highly migratory Latino subgroup, Puerto Rican women. Using pooled origin-destination data from surveys conducted in Puerto Rico and in the New York metropolitan area, we find that first- and second-generation migrants to the U.S. mainland face substantially higher risks of conceiving and bearing a first child before marriage than do nonmigrants in Puerto Rico. This pattern is due largely to the relatively early transition to sexual activity among mainland women. Given the negative long-term consequences of premarital childbearing for women and their children, our findings call into question the assumption that migrants necessarily experience only positive outcomes as a result of the assimilation process.