Concern about high infant mortality and morbidity in the United States, combined with the erosion of private insurance coverage, sparked major expansions in the Medicaid program in the 1980s. This study examines how the Medicaid expansions for pregnant women affected access to prenatal care for low-income women through case studies conducted in four states early in 1991. Despite the significantly greater share of births covered by Medicaid in the period 1986 to 1991, the timely initiation of prenatal care improved in only one state. Although prenatal services increased in some areas, significant problems persisted in others. The growth in capacity of the prenatal care system was greatest when state and local policies designed to increase supply were also instituted. While the Medicaid expansions eliminated significant barriers to prenatal care for low-income women, other policies that have been designed to reduce the remaining barriers may be necessary in order significantly to expand access to prenatal care and to improve birth outcomes.