Improving pregnancy outcomes for low-income women has been a longstanding Medicaid objective, yet exceptionally low Medicaid participation rates for private practice obstetrician-gynecologists (OB-GYNs) suggest that access to maternity care may be particularly limited. Using a national sample of more than 2,800 office-based physicians, the authors analyzed the factors influencing the Medicaid participation decision of physicians in three specialties: OB-GYN, pediatrics, and general surgery. Regression results suggest that OB-GYNs are equally, or even more, sensitive to Medicaid reimbursement and program administration characteristics. Higher Medicaid fees definitely raise OB-GYN participation rates, for instance. OB-GYNs are also more willing to participate in those states where Medicaid programs are more generous in their eligibility requirements and where administrative red tape is less onerous.