This article examines first-birth timing among Mexican women in the United States over two birth cohorts. Currently, Mexican women are one of a small group that maintains above-replacement fertility in the United States, contributing to both Mexican population growth and overall national population growth. Yet, the fertility timing of Mexican women has undergone a significant transformation over the past 40 years. Using the 1988, 1995, and 2002 National Survey of Family Growth, this article employs first-birth probabilities and Cox proportional hazard models to show how Mexican fertility timing differs from that of other major racial and ethnic groups. The evidence suggests that the racial/ethnic, nativity, and educational diversity of the Mexican-origin population has led to the emergence of two distinct patterns of first-birth timing. The findings from this study point to the need to reconsider the role of Mexican fertility timing in important national discussions of women's reproductive health and immigration/population polices. (Contains 3 tables and 4 figures.)