We bought these bottles because they were supposed to be the closest to breast feeding. I was planning on bottle and breastfeeding. These bottles leaked all over my baby and our rugs.We had to change the soaked outfits in the middle of the feedingand put more formula in the bottle. ( all in the middle of the night when we were half asleep...Also, my son had the worst gas because he was sucking and sucking and wasn't getting only small amounts of formula and probably air..It was taking him 45 minutes to drink 3 ozs.. We switched to Dr Browns since my son has Reflux and have been so happy ever since. Of course after I asked around, everyone else had the same problems with the Advent- now they tell me! I'm here to tell everyone, don't buy these bottles. Advent should go back to the drawing board and fire the product developers who worked on this!