This study measured the prevalence of inadequate prenatal care and identified associated factors using different criteria in postpartum women in Rio Grande, Rio Grande do Sul State, Brazil, in 2010. A standardized questionnaire was applied within 24 hours after delivery to all mothers of children born in the two local hospitals. We used the chi-square test to compare proportions and Poisson regression with robust variance in the multivariate analysis. The study interviewed 2,395 mothers (97.2% of the total). The rates of inadequate prenatal care were 28%, 27%, and 58% according to the criteria proposed by Takeda, Coimbra et al., and Silveira et al., respectively, with large differences across categories. Poor mothers showed the highest prevalence ratio for inadequate prenatal care. After adjustment, nearly all the model's variables were significantly associated with inadequate care according to the Takeda and Coimbra et al. criteria, but few were associated when the Silveira et al. criteria were used. The study showed that the criteria proposed by Silveira et al. were more robust, and that it is necessary to improve quality of prenatal care, especially for poor mothers.