Babies under 6 months old should not drink plain water because their kidneys can’t handle it, and even small amounts can dangerously dilute the sodium in their blood. Breast milk and formula already contain all the water a baby needs. Giving extra water can lead to a rare but serious condition called water intoxication, which can cause seizures.
How Water Disrupts an Infant’s Body
The core danger comes down to sodium balance. When a young baby takes in plain water, it dilutes the sodium concentration in their bloodstream. This imbalance causes water to rush from the blood into cells, making them swell. In the brain, this swelling happens primarily in supportive cells called glial cells, which absorb the excess water to try to protect nerve cells. If the sodium drop happens quickly (within 48 hours), the result is brain swelling that can become life-threatening.
Adults handle extra water easily because their kidneys rapidly flush it out. Newborns and young infants don’t have that ability. Their kidneys are still maturing and have a limited capacity to either concentrate or dilute urine in response to fluid changes. That means when excess water enters a baby’s system, the kidneys can’t clear it fast enough, and sodium levels plummet.
How Small a Baby’s Stomach Really Is
A newborn’s stomach on day one is roughly the size of a marble. By day three, it’s about the size of a ping-pong ball. By day ten, it’s comparable to an extra-large chicken egg. Even by three or four months, a baby’s stomach holds only about 4 ounces per feeding.
This tiny capacity is the second reason water is harmful: it takes up space that should go to breast milk or formula. Every ounce of water a baby drinks is an ounce of nutrition they’re missing. Over time, this displacement can lead to poor weight gain and inadequate development. The same logic applies to over-diluting formula with extra water. Adding too much water to formula reduces the nutrient concentration per bottle, which can cause failure to thrive and poor growth.
Signs of Water Intoxication
Water intoxication in infants is uncommon, but it does happen, particularly when caregivers dilute formula to stretch it or offer water bottles to very young babies. The CDC has documented cases of seizures in infants who were given commercial bottled water as a supplement to feedings.
The early signs include unusual irritability or excessive sleepiness, which can be easy to dismiss as normal fussiness. More concerning symptoms include low body temperature, puffiness or swelling (especially around the face), and seizures. If a baby who has consumed plain water becomes unusually drowsy or difficult to wake, that’s an emergency.
What About Hot Weather?
Parents often worry about dehydration during heat waves, and it feels instinctive to offer water. But for babies under 6 months, the answer is simply more breast milk or formula. Breast milk naturally adjusts its water content during hot weather, so more frequent nursing sessions keep a baby hydrated without the risks of plain water. Formula-fed babies can be offered more frequent bottles of properly mixed formula.
When Water Becomes Safe
At around 6 months, babies can start having small amounts of water. This timing lines up with the introduction of solid foods, when babies begin needing sips of water to help with digestion and to get used to drinking from a cup. The CDC recommends 4 to 8 ounces of water per day for babies between 6 and 12 months. That’s roughly half a cup to one cup spread across the entire day, not per sitting.
At this age, breast milk or formula still provides the majority of a baby’s hydration and nutrition. Water is a complement, not a replacement. There’s no need to push water intake; small sips with meals are enough. Once a baby reaches 12 months and transitions toward a more varied diet, water becomes a bigger part of daily hydration.
Common Mistakes to Avoid
- Stretching formula with extra water. Always follow the mixing instructions on the label exactly. Over-dilution reduces calories and nutrients per bottle, and it introduces the same sodium-dilution risk as giving plain water.
- Offering water for hiccups. Hiccups are normal in infants and resolve on their own. A small amount of breast milk is fine if you want to try something, but water is not the answer for babies under 6 months.
- Using water to soothe between feedings. If a baby seems hungry between feedings, they likely need more milk or formula, not water.
- Giving water during illness. Sick babies under 6 months need more frequent breastfeeds or formula, not water. If you’re concerned about dehydration from vomiting or diarrhea, a pediatrician may recommend an oral rehydration solution, which contains the right balance of electrolytes.