Distrust and dissatisfaction with contraceptive methods among low-income Brazilian women contributes to incorrect contraceptive use, harmful health risks, and an extensive reliance on surgical sterilization. Data from ethnographic fieldwork in a low-income neighborhood in urban Brazil illustrate that women's concerns regarding contraception make sense in the context of their general health and illness beliefs and their understanding of reproductive physiology. Women interpret and experience biomedical concepts and contraceptive methods (such as “hormones” and oral contraceptives, respectively) according to an ethnophysiology of fecundity, menstruation, and conception. These popular representations of reproduction embody social relations and local experience and therefore persist despite the influence of biomedical models. Recognition of this ethnophysiology and greater emphasis on culturally appropriate counseling and education among health care professionals will improve women's experience and effective use of reversible contraception.