The conceptions that prospective primary teachers (PPTs) have about breastfeeding (BF) were explored as part of their training in school content related to health and the human body. For this, the PPTs answered a questionnaire designed ad hoc, without having received any prior training about the topic. The questionnaire had previously been validated by a panel of experts made up of midwives and researchers in science education. The results show that the PPTs have poorly informed ideas about both the biological processes associated with the genesis of breastmilk and the effects of BF on the mother and infant. Those ideas largely coincide with those of the rest of society (e.g., believing that there are women who produce milk that has insufficient nutritional value for their infants). The results are discussed with the goal of improving PPTs' scientific-didactic training about BF as a topic of scientific, social and environmental importance.