evidence: The American Academy of Pediatrics guidelines say water can be introduced at around six months. That’s a simple answer, but it doesn’t tell us why. The primary reason to avoid water for babies is that it displaces nutrition. Up to a year, babies get a lot of their calories through liquid, either breast milk or formula. (These are both almost 90% water, so babies are, in fact, getting plenty of water.) If your baby started drinking just water, it could easily displace some of their breast milk or formula intake, which is a potential problem. [...] You want to avoid water under 6 months primarily because baby’s kidneys can’t handle a fluid that is not isotonic (same concentration) as their blood. The baby’s kidneys lack the maturity to adjust the body’s sodium concentration and adding water makes you risk having seizures because your sodium levels drop. This is why it’s important not to water down formula too. Formula (when mixed correctly) and breast milk both have the right level of sodium for a baby’s body. (From a pediatrician who talks a lot about water with parents!)