BACKGROUND: Teens and racial and ethnic minority women are less likely to initiate prenatal care (PNC) in the first trimester of pregnancy than their counterparts. OBJECTIVE: This study examines the impact of Medicaid program changes in the late 1990s on the timing of Medicaid enrollment and PNC initiation among pregnant teens by race and ethnicity. RESEARCH DESIGN: Using Medicaid enrollment and claims data and a difference-in-differences method, we examine how the patterns of prepregnancy Medicaid enrollment, PNC initiation, and racial and ethnic disparities in PNC changed over time after controlling for person- and county-level characteristics. SUBJECTS: We included 14,089 teens in Florida with a Medicaid-covered delivery in fiscal years 1995 and 2001. MEASURES: Prepregnancy enrollment was defined as enrollment 9 or more months before delivery; late or no PNC was defined as initiation of PNC within 3 months of delivery or not at all. RESULTS: For teens enrolled in traditional welfare-related categories, the proportion with prepregnancy Medicaid enrollment increased and the proportion with late or no PNC declined from 1995 to 2001. Teens enrolled under the Omnibus Budget Reconciliation Act (OBRA) expansion category in 2001 were less likely than welfare-related teen enrollees to have prepregnancy coverage but were more likely to initiate PNC early. Racial disparities were found in PNC initiation among the 1995 welfare-related group and the 2001 expansion group but were eliminated or greatly reduced among the 2001 welfare-related group. CONCLUSIONS: Providing public insurance coverage improves access to care but is not sufficient to meet Healthy People 2010 goals or eliminate racial and ethnic disparities in PNC initiation.