Our objective was (1) to identify the subgroup of women most affected by the regulatory change expanding Tennessee Medicaid eligibility for pregnant women from 45% of the federal poverty level to 100% and (2) to examine whether increased enrollment correlated with greater use of prenatal care and improved reproductive outcomes. We linked Tennessee birth and fetal death certificates to Medicaid enrollment files. We compare outcome rates in the 12-month period before the change in the Medicaid regulations with similar rates for the 10-month period after the change had been in effect nine months. We found the increase in Medicaid enrollment that occurred after the expansion was greatest for teenage mothers. Among teens, Medicaid enrollment increased 18%, and the odds of receiving no prenatal care or only late (third-trimester) care were reduced 16% (95% confidence interval = 8%, 24%) after we controlled for potential confounders. However, there was no improvement in first-trimester use of prenatal care or in birth outcomes. This finding suggests the need to evaluate carefully subsequent regulatory changes, which sought to promote early prenatal care by removing barriers to early Medicaid enrollment in pregnancy.