Despite the well-known benefits of breastfeeding, breastfeeding rates remain suboptimal, particularly for women with lower socioeconomic position. Although popular, breastfeeding apps are often poor quality; their impact on breastfeeding knowledge, attitudes, confidence and intentions is unknown. A mixed method prepost feasibility study was conducted to: 1) explore the feasibility of the My Baby Now app in providing perinatal breastfeeding support; 2) examine the impact on breastfeeding knowledge, attitudes, confidence and intentions; 3) to examine any differences in acceptability and impact of the app according to maternal education.The My Baby Now app was offered to pregnant women 20-30 weeks gestation.Breastfeeding knowledge and intentions were collected at baseline (T1) and 36-38 weeks gestation (T2); attitudes and confidence were collected at baseline, T2 and T3 (8-12 weeks post-partum). App engagement was measured via app analytics.Qualitative interviews were conducted with a purposeful sample following T3. Of 266 participants recruited, 169 (64%) completed T2 and 157 (59%) completed T3.Mothers without university education rated the app to be higher quality, more useful and impactful than mothers with university education. From T1-T2, breastfeeding knowledge (59.6% vs. 66.5%, p < 0.001) and exclusive breastfeeding intentions (76.6% vs. 80.9%, p < 0.001) increased. Breastfeeding attitudes and confidence scores also increased significantly across T1-T2 and T1-T3. App engagement during pregnancy predicted changes in breastfeeding attitudes from T1-T2 among participants without university education. App engagement did not predict changes in breastfeeding knowledge, confidence or intentions. Future randomised controlled studies should examine the effectiveness of mHealth interventions on breastfeeding outcomes.