evidence: Yes, but not of the H2O variety. Your little one — if under 6 months old — should be receiving both nutrition and hydration from breast milk or formula, not water. [...] So giving a baby younger than 6 months even a moderate amount of water in a short period of time can lead to hyponatremia, which at its most dangerous can cause brain swelling and even death. In fact, because the brain is still developing as well, the swelling can happen more easily in an infant with hyponatremia than in an adult with hyponatremia. Remember: Tiny tummy + immature kidneys + developing brain = Avoid giving babies water until they are 6 months of age