The persistence of negative perinatal outcomes in Rio de Janeiro suggests problems in the quality of prenatal care. The most recent study in the city showed that only 38% of prenatal care was adequate. This study aimed to evaluate the adequacy of prenatal care under the Brazilian Unified National Health System in the city of Rio de Janeiro. A cross-sectional study in 2007-2008 interviewed 2,422 women receiving prenatal care for low-risk pregnancy. Evaluation of care used the PHPN index, based on guidelines from the Program for Humanization of Prenatal Care and Childbirth (Brazilian Ministry of Health) and an expanded PHPN index, which included clinical-obstetric procedures, prescription of supplementary ferrous sulfate, and educational activities. According to the PHPN index, 38.5% of prenatal care was adequate, as compared to 33.3% based on the expanded PHPN index. Strategies to expand early entry of pregnant women into prenatal care and better use of their contact with the health services in order to promote healthcare measures are essential to correct this situation.