Hey everyone, My baby is 4 months old and we use the Avent bottles. I'm just a bit confused as to when we should switch to the next level of the nipple flow. We use the level 3 as of right now. He seems to be ok with it. But I remember reading some where that you have to switch out the nipples every 3 months or so. Also when he drinks he's milk sometimes he sucks so hard that the nipple sucks in. As if he is sucking to hard. Not sure if that was useful information but I thought I would throw it our there. Thanks everyone!