Objective: To explore relationships among breastfeeding knowledge, breastfeeding confidence, and infant feeding plans and their effects on feeding practices in first-time breastfeeding mothers. Design: Prospective descriptive design. Setting: Telephone interviews were conducted prenatally and at 6 weeks, 3 months, and 6 months postpartum. Participants: Seventy-four of 83 first-time mothers with prenatal intentions to breastfeed completed all study requirements. The majority were White (95%), between the ages of 21 and 30 years (73%), with a post-high school education (85%), and household incomes of more than 200% of the federal poverty guideline (88%). Main Outcome Measures: Breastfeeding knowledge, breastfeeding confidence, planned infant feeding method, planned breastfeeding duration, weeks of daily human milk substitute feeding, breastfeeding duration, achievement of breastfeeding goals. Results: Breastfeeding knowledge was strongly correlated with breastfeeding confidence (r =.262; p =.025) and actual lactation duration (r =.455; p =.0001). Compared with women planning to exclusively breastfeed their infants, those planning to combination feed planned shorter breastfeeding duration (p =.022), reported shorter actual duration (p =.004), and were less likely to meet their breastfeeding goal (p =.03,4). The variables maternal education, breastfeeding knowledge, and weeks of daily human milk substitute feeding were used to develop a prediction equation that correctly categorized 93% of participants who met their breastfeeding goal and 90% of those who did not. Conclusions: Expectations and the actual breastfeeding experience differed among women planning to combination feed and those planning to exclusively breastfeed. Whether a cause or consequence, daily human milk substitute feeding was associated with negative breastfeeding outcomes.