Background <p>Gestational anemia remains an important public health problem, especially in developing countries, and is associated with adverse maternal and perinatal outcomes. Prophylactic iron supplementation during pregnancy is a simple and effective strategy to prevent this condition and is routinely recommended during prenatal care. In Brazil, supplements are provided free of charge through the public health system; however, adherence to treatment remains variable. This study aimed to identify the sociodemographic and healthcare-related factors associated with inadequate iron supplementation among pregnant women receiving care in the public health system.</p> Methods <p>This cross-sectional study was conducted between March 2023 and May 2024 at the Maternity Unit of the Irmandade da Santa Casa de Misericórdia de São Paulo, a quaternary teaching hospital located in São Paulo, Brazil, and a reference center for obstetric care within the Brazilian Unified Health System (SUS). Women aged ≥ 18 years who were evaluated within 24&#xa0;h after delivery were eligible. The exclusion criteria included fetal death, multiple pregnancies, and the presence of major fetal malformations. Data, including sociodemographic information, clinical history, prenatal care characteristics, and iron supplementation during pregnancy, were obtained through a structured questionnaire administered during the immediate postpartum period.</p> Results <p>A total of 381 patients were included. Overall, 176 women (46.2%) had adequate iron supplementation during pregnancy and 205 (53.8%) had inadequate supplementation. Among the variables analyzed, drug use during pregnancy was the only sociodemographic factor that was significantly associated with inadequate iron supplementation (<i>p</i> = 0.001 crude OR 2.93, 95% CI 1.57–5.48). A trend toward greater adherence was observed among pregnant women followed exclusively within the public health system than among those receiving private, mixed, or overseas prenatal care, although this difference was not statistically significant. In multivariable logistic regression, self-reported drug use (aOR 2.54, 95% CI 1.33–4.87; <i>p</i> = 0.005), non-use of other vitamins during pregnancy (aOR 6.00, 95% CI 2.45–14.72; <i>p</i> &lt; 0.001) and inadequate prenatal care (aOR 0.49 for adequate vs. inadequate prenatal care, 95% CI 0.25–0.94; <i>p</i> = 0.033) remained independently associated with inadequate iron supplementation.</p> Conclusions <p>Inadequate iron supplementation during pregnancy is associated mainly with social and behavioral vulnerability, particularly drug use. These findings suggest that adherence depends not only on the availability of supplements but also on the quality of prenatal care and professional guidance. Public health strategies aimed at health education, strengthening primary care, and longitudinal follow-up of pregnant women may contribute to improving adherence and reducing the prevalence of gestational anemia.</p>