Background/Objective: Persistent instability in insurance coverage before and after pregnancy among low-income mothers in the United States contributes to delayed prenatal care and poor infant outcomes. States that expand Medicaid under the Affordable Care Act (ACA) make public insurance free for many low-income women regardless of parental or pregnancy status. Our objective is to analyze the effects of expanding Medicaid in Ohio on enrollment of pregnant women and receipt of recommended prenatal care. A key objective in the state is to address infant mortality as Ohio ranks above the national average and racial disparities persist. Materials and Methods: We used linked enrollment/claims/birth certificate data for women with Medicaid-paid deliveries/births, aged 19-44 years with months of last menstrual period (LMP) in calendar year 2011-2015 (N = 290,091). We used interrupted time-series analysis of enrollment prepregnancy and receipt of guideline-concordant screenings (anemia, asymptomatic bacteriuria, chlamydia, human immunodeficiency virus [HIV], and TORCH) and prenatal vitamins after versus before the expansion. We stratified by parity since first-time mothers would be impacted more. Results: We found almost a 12 percentage point increase in enrollment prepregnancy among first-time mothers compared with almost a 6 percentage point increase for parous women. We found significant increases in all screens and vitamins for both groups. TORCH screening increased 8 percentage points and receipt of prenatal vitamins almost 14 percentage points, by the end of 2015 for first-time mothers, compared with 5 and 4 percentage points, respectively, for parous women. Conclusions: Early enrollment and prenatal care for low-income women in Ohio could erode if the state's Medicaid expansion is altered.