I purchased 12 of these bottles for my daughter to have at daycare after I went back to work. She was breastfed and she had no problem using these bottles! I kept the newborn nipple on them, and still use the newborn nipple now even though she is 7 months old, as recommended by my lactation consultant. These bottles only leaked for me if the ring was not used, which I did forget to use once. Other than that one time, the bottles have never leaked and I've been using them for months. Great buy!!