when they suddenly start taking too long to finish the bottle, or loose interest in the middle, and start looking around, or gets frustrated.I thought my little guy was ready to switch up, so i tried it and he was crying bc it was coming out too fast and getting all over his face. so just look out for that. just in case u think your lo is ready but isnt. good luck
Same thing happened to us. However, I decided a better way to determine which size is best- cut a crisscross in the "old" nipple. DS preferred this better.
Re: Question for bottle feeders...