Giving water to a baby under 6 months old can be dangerous. Even small amounts can dilute the sodium in their blood, a condition called water intoxication, which can cause seizures, abnormally low body temperature, and swelling. Babies get all the hydration they need from breast milk or formula, and their tiny bodies simply aren’t built to handle extra water.
Why Babies Can’t Handle Water
A newborn’s kidneys are still developing. Although kidney formation finishes before birth, the organs continue maturing well into infancy. In the first week of life, a baby’s kidneys receive only about 10% of cardiac output, compared to a much larger share in adults. They also can’t concentrate urine the way adult kidneys do. This means when extra water enters a baby’s system, their kidneys can’t flush it out efficiently, and sodium levels in the blood drop.
Stomach size is the other factor. On day one of life, a newborn’s stomach holds roughly 5 to 7 milliliters, about the size of a marble. By day 10, it grows to around 60 to 80 milliliters, roughly the size of a large egg. Even by three or four months, stomach capacity tops out at about 4 ounces per feeding. Filling any of that limited space with water means the baby takes in less breast milk or formula, missing out on the calories, fat, protein, and electrolytes they need to grow.
Water Intoxication in Infants
Water intoxication happens when excess water dilutes the sodium in a baby’s bloodstream, a condition called hyponatremia. Sodium is essential for nerve and muscle function, and when levels drop too low, the brain is affected first. The CDC has documented cases of hyponatremic seizures in infants who were fed plain bottled water. Symptoms include unusual irritability or extreme sleepiness, low body temperature, facial or body swelling, and seizures.
This doesn’t require a dramatic amount of water. Because babies are so small, even a few ounces can shift their electrolyte balance significantly. The risk is highest in the first six months, when babies rely entirely on milk or formula for both nutrition and hydration.
The Risk of Diluting Formula
One common way babies end up with too much water is through over-diluted formula. Some parents stretch formula by adding extra water, sometimes to save money or because they believe it will help with constipation. This is harmful for two reasons: the baby gets excess water their kidneys can’t process efficiently, and they miss out on the nutrients they need. Over-diluted formula has been linked to failure to thrive and poor growth. Always follow the mixing instructions on the formula label exactly.
When Water Becomes Safe
Around 6 months, when babies start eating solid foods, you can begin offering small sips of water. The recommended amount for babies between 6 and 12 months is 4 to 8 ounces per day, roughly half a cup to one cup. This isn’t meant to replace any breast milk or formula feedings. It’s a small supplement to help with digestion of solid foods and to get your baby used to drinking from a cup.
An open cup, sippy cup, or straw cup all work well for this. There’s no need to offer water between every feeding or to push your baby to finish a set amount. A few sips here and there throughout the day is plenty at this stage. Breast milk or formula remains the primary source of hydration and nutrition through the entire first year.
What About Hot Weather or Illness?
Parents often worry about dehydration during summer heat or when a baby has a fever. For babies under 6 months, the answer is still more breast milk or formula, not water. Both are already mostly water and provide hydration along with the electrolytes and calories a baby needs. Offering extra nursing sessions or slightly more frequent bottles is the safe way to keep a young baby hydrated in hot weather or during illness.
For babies over 6 months who are eating solids, you can offer a bit more water than usual on hot days, but breast milk or formula should still be the main source of fluids. If your baby shows signs of dehydration, like fewer wet diapers, a dry mouth, or no tears when crying, increasing milk or formula feedings is the first step.
Signs Something Is Wrong
If a baby has consumed a significant amount of plain water and begins acting unusually sleepy or irritable, feels cool to the touch, or develops puffiness in the face or body, these are warning signs of water intoxication. Seizures are the most serious symptom. This is a medical emergency that requires immediate attention, as the brain swells when sodium drops too low. The younger and smaller the baby, the less water it takes to cause a problem.