The objective of this study was to analyze the exercise of competences in prenatal care performed by nurses in the East Zone of the city of São Paulo through the identification of the activities performed by them and their frequency as well as possible difficulties found. This quantitative, descriptive-exploratory study was carried out in 59 health centers, from October 2006 to December 2007, with a sample of 131 nurses. The results showed that nurses did not perform essential competences necessary to obtain a qualified prenatal care due to personal and institutional barriers found in their job. It was concluded that there is a need to review the policies of public health structures in São Paulo to guarantee the implementation of the Unique Health System guidelines regarding the improvement of prenatal care and to provide human and financial resources to reach this purpose.