The scope of this study was to analyze the content of prenatal care in São Luís, Maranhão, Brazil, and the factors associated with its inadequacy. A cross-sectional study was conducted based on data from the birth cohort of São Luís in 2010. The content of prenatal care was defined as inadequate when it did not meet the criteria of the Program for Humanization of Prenatal and Delivery Care, which establishes early initiation of prenatal care, minimum number of medical consultations, basic laboratory tests, tetanus vaccination and obstetric procedures. Poisson regression was used to observe associations of the variables with the outcome. The inadequacy rate was high (60.2%). The variables associated with inadequacy were: class C socioeconomic status (PR = 1.39; CI = 1.26-1.55); class D/E socioeconomic status (PR = 1.60; CI = 1.43-1.79); unqualified/unemployed mother (PR = 1.24; CI = 1.11-1.37); 5-8 years of schooling (PR = 1.12; CI = 1.06-1.19); 0-4 years of schooling (PR = 1.13; CI = 1.01-1.26); not being religious (PR = 1.10; CI = 1.04-1.17); alcohol use during pregnancy (PR = 1.13; CI = 1.06-1.20), and being attended by the public service (PR = 1.75; CI = 1.54-2.00). The results showed inadequacy and inequality of prenatal care, revealing that women of lower socioeconomic status received lower quality care.