BACKGROUND: Although inadequate prenatal care has been associated with adverse perinatal outcomes, reports on the factors associated with poor prenatal care in developing Latin American countries are scarce. OBJECTIVE: To determine factors associated with inadequate prenatal care among women from low socioeconomic circumstances. METHOD: Women delivered after a pregnancy duration of more than 20 weeks at the Enrique C. Sotomayor Obstetrics and Gynecology Hospital, Guayaquil, Ecuador, were surveyed. The questionnaire collected sociodemographic data and reasons for having inadequate prenatal care. Adequacy of prenatal care was measured with the Kessner index and correlated to the sociodemographic data. RESULTS: During the study period, 1016 pregnant women were surveyed. Among them, there were adolescents (23.7%), primigravidas (30.8%), and women with a high-risk pregnancy (29.3%). According to the Kessner index, prenatal care was considered adequate or inadequate in 24.5% and 75.5% of cases, respectively. Knowledge regarding the importance of adequate prenatal care and the effects of poor prenatal care was lower among women who had received inadequate prenatal care. The women that were considered to have had adequate prenatal care had at least one visit, and they were more often cared for by a specialist than women who considered having inadequate prenatal care. The three most important reasons associated to inadequate prenatal care in this series (n=767), were economic difficulties having to care for a small child, and transportation difficulties. Logistic regression analysis determined that women with undesired pregnancies who resided in rural areas and were para 5 or higher had an increased risk of inadequate prenatal care. On the other hand, an adverse outcome to a prior pregnancy (abortion, intrauterine fetal demise, or ectopic pregnancy) decreased this risk. Marital status and educational level were confounding factors. CONCLUSIONS: Although prenatal care at our institution is free, adequacy was thought to be low. The main factors associated with poor prenatal care were mostly conditions related to poverty.