My baby is 13 months old and is drinking 3 bottles a day, 5 ounce each time. She is a very good eater with a varied diet. She uses a sippy cup- different types including one with a straw - to take water with meals. I have recently tried dropping the afternoon bottle and replacing it with milk. She will drink some but not never the entire amount. She is getting quite constipated and struggling to go when she does. When I give her the 3 bottles she's fine. Is this just a developmental thing and she'll eventually get better with the cup and drink more? Don't want to drop bottles and then this leads to her not hydrated enough. Any advice?