Breastfeeding offers significant benefits to the breastfed infant as well as the breastfeeding woman. The World Health Organization now recommends exclusive breastfeeding until six months, followed by supplementation and continued breastfeeding to two years or more. Around the world, public health programs endeavour to promote breastfeeding through educational programs. In Brazil, such programming is widespread, and yet less than 30% of women in São Paulo breastfeeding exclusively even to four months post-partum. This study uses a qualitative-quantitative bio-experiential approach to explore the way that stressful experiences and circumstances in the lives of low-income women from the Eastern Zone of São Paulo, Brazil, influence their decision to wean or supplement their infant before 12 weeks post-partum. Sixty-five first-time mothers participated in a 12-week longitudinal study of life stressors and breastfeeding practice. Participants were asked to complete one pre-partum and six post-partum interviews. Narrative and biological data were collected from each participant at each interview. Statistical analysis revealed that among these participants the breastfeeding hormone oxytocin did not mediate breastfeeding duration. Oxytocin appeared to act as a biomarker of stressful experience, while Epstein-Barr Virus antibody titre, a commonly used biological measure of psychosocial stress, did not. Unplanned pregnancy, older age and higher mean oxytocin level were statistically associated with weaned outcome at 12 weeks. Unplanned pregnancy, older age, higher mean oxytocin level, higher mean satisfaction score regarding financial situation and lower mean satisfaction score regarding interpersonal factors were associated with decreased duration of any breastfeeding. Unplanned pregnancy, older age and lower mean satisfaction score regarding interpersonal factors were associated with decreased duration of exclusive breastfeeding. Ethnographic analysis revealed that the effect of unplanned pregnancy may be connected to the discourse of the self-sacrificial, child-centric “good mother.” Exclusive breastfeeding was seen as a hallmark of this idealised maternal type. Single women with unplanned pregnancies expressed a great deal of ambivalence towards their own maternity and toward the somewhat unobtainable good mother ideal, especially with relation to the physical and psychological challenges breastfeeding. Women’s ambivalence appeared to influence their decisions to supplement or wean their infants by or before 12 weeks post-partum.