Objectives To determine whether expanding Emergency Medicaid to cover prenatal care in Oregon affected maternal health outcomes for unauthorized immigrants. Methods This study takes place in Oregon from 2003 to 2015 and includes all Emergency Medicaid and Medicaid claims for women aged 12–51 with a pregnancy related claim. To isolate the effect of expanding access to prenatal care, we utilized a difference-in-differences approach that exploits the staggered rollout of the prenatal care program. The primary outcome was a composite measure of severe maternal morbidity and mortality. Additional outcomes include adequacy of prenatal care, detection of pregnancy complications and birth outcomes. Results A total of 213,746 pregnancies were included, with 35,182 covered by Emergency Medicaid, 12,510 covered by Emergency Medicaid Plus (with prenatal care), and 166,054 covered by standard Medicaid. Emergency Medicaid Plus coverage did not affect severe maternal morbidity (all pregnancies 0.05%, CI − 0.29; 0.39; high-risk pregnancies 2.20%, CI − 0.47; 4.88). The program did reduce inadequate care among all pregnancies (− 31.75%, 95% CI − 34.47; − 29.02) and among high risk pregnancies (− 38.60%, CI − 44.17; − 33.02) and increased diagnosis of gestational diabetes (6.24%, CI 4.36; 8.13; high risk pregnancies 10.48%, CI 5.87; 15.08), and poor fetal growth (7.37%, CI 5.69; 9.05; high risk pregnancies 5.34%, CI 1.00; 9.68). The program also increased diagnosis of pre-existing diabetes mellitus (all pregnancies 2.93%, CI 2.16; 3.69), hypertensive diseases of pregnancy (all pregnancies 1.28%, CI 0.52; 2.04) and a history of preterm birth (all pregnancies 0.87%, CI 0.27; 1.47). Conclusions for Practice Oregon’s prenatal care expansion program produced positive effects for unauthorized immigrant women and their children.