Introduction: Some mothers do not comply with breastfeeding due to unawareness of the practice. Aims: To characterize the demographic and obstetrical profile of postpartum women and to verify the influence of their instruction level and the number of prenatal consultations on breastfeeding knowledge. Design: Descriptive and quantitative study. Method: Individual interviews were carried out with 150 postpartum women at a public maternity rooming-in. The data were processed in the SPSS, version 13.0. Results: Most women were between 18 and 35 years old, less than eight schooling years; 91 (60.7%) were multiparae and 87 (58%) had not breastfed previously. Increased knowledge was verified on breastfeeding protection against ovarian and breast cancer (n=66; 89.2%), child development and promotion of mother and baby bonds (n=74; the 100%) and protection against child infections (n=73; 98.6%) among postpartum women with eight schooling years. The largest number of prenatal consultations positively influenced the understanding of maternal milk as an optimum food for children (n=109; 100%), the baby's bottle use to worsen breast suckling (n=88; 80.7%), the introduction of other foods affecting the child health (n=93; 85.3%) and painless breastfeeding of mothers who follow correct handling (n=74; 67.9%). Conclusion: The study reveals that factors such as instruction and number of prenatal consultations influence the knowledge level on breastfeeding, therefore reinforces the need for prenatal care promotion and educative approaches toward breastfeeding. Relevance for the clinical practice: This study brings to attention factors related to mother’s knowledge about breastfeeding. Thus, women awareness must be raised toward exclusive breastfeeding as the best health practice to the mother-baby binomial.