It’s a bright, sunny day outside, and your whole family is feeling the heat and guzzling water. Your newborn surely needs some hydration, too, right?

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.

You probably know this, but you might not know why. It’s because babies’ bodies aren’t suited for water until several months after birth. Tiny tummies and developing kidneys put them at risk for both nutrient loss and water intoxication. Here’s the scoop.

Baby tummies are quite small. In fact, at birth, a baby’s belly only holds about 1 to 2 teaspoons, or 5 to 10 milliliters (mL)! Clearly, it does empty fast — which is why your babe needs so many feedings in a 24-hour period — but you want to fill that little tummy with nutrient-rich breast milk or formula.

So it makes sense that one risk of giving your baby water is that you’ll be filling their belly with a really quite useless substance (at least to a baby) and leaving no room for those vitamins, minerals, fat, and calories so crucial for growth and development. This can cause serious problems.

Baby’s tummy does grow over the first 6 months of life, but it’s pretty gradual. By the time they’re 1 month old, their stomach capacity is about 2.7 to 5 ounces (80 to 150 mL). By 6 months — when you can introduce little sips of water — they can generally hold about 7 ounces (207 mL) at a time.

Even between 6 months and 1 year of age, the amount of water you give your baby should be very limited. It’s more for them to get the taste and experience of water rather than for any real medical purpose like hydration. After all, formula and breast milk are very hydrating — and also give your little one what they need to grow and thrive.

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Another very serious risk of giving babies water before they’re ready is water intoxication.

Hold the front door. Water — toxic?

Absolutely. In fact, water can be toxic to anyone if drunk in large quantities. But unsurprisingly, “large” is very relative to size and age here. An adult with healthy kidneys, for example, would have to drink several liters in a short period of time to get to the point of water intoxication.

That said, it does happen to people, particularly soldiers and athletes, who tend to be in situations where they can become dehydrated quickly and then overcompensate.

In short, when the kidneys are given more water than they can handle, the excess water ends up in your bloodstream. This dilutes the fluid in your bloodstream and lowers the concentration of important electrolytes, like sodium. Too much dilution and you’re at risk for hyponatremia, which literally means too little (hypo) salt in the blood (natremia).

And baby kidneys can’t handle as much water as adult kidneys — not by a long shot. In addition to being much smaller than an adult’s kidneys, a baby’s kidneys are also not as developed. So they can’t process as much water at a time.

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.

A dangerous equation

Remember: Tiny tummy + immature kidneys + developing brain = Avoid giving babies water until they are 6 months of age

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The thing is, most parents aren’t filling bottles with water and giving them to their infants.

The risk comes from things that you might not even give a second thought.

For example, while many swimming schools don’t offer lessons to babies under 6 months, some will start them as young as 4 months. There’s nothing inherently wrong with introducing a baby to the pool if it’s done safely — but without the proper precautions, babies can swallow pool water and experience water intoxication as a result.

Another seemingly harmless act that can lead to trouble is diluting formula or breast milk. Going back to our hydration scenario, it might seem to make sense to mix more water into your baby’s formula powder on a hot day. But don’t do this — it deprives baby of nutrients and can also lead to them getting more water than their kidneys can handle.

Because formula and breast milk are calorie rich, they stay in the body longer rather than overwhelming the kidneys. As a nice side effect, staying in the body longer also means they’re good at keeping your little one hydrated — no extra water needed.

At around 6 months of age, it’s OK to introduce small amounts of water — we’re talking on the teaspoon or tablespoon scale, not the full-bottle scale. It’s a good time to start introducing the concept that thirst can be quenched with water, but your baby’s main source of hydration (not to mention nutrition) should continue to be breast milk or formula.

Most babies will see water as a sort of novelty at this age and still prefer their milk. Some might even balk at the taste and make a face, especially if they were expecting something else! That’s OK — this will change.

By 1 year old, your baby — who’s just about a toddler, if you can believe it! — can have water in larger quantities as they want it, along with cow’s milk and a nutritious diet.

Related: When can baby drink water?

Talk to your pediatrician if you have any concerns about your baby’s hydration or their readiness for water. Depending on if your baby was born prematurely or has certain health conditions, your timeline for introducing water may vary.

In addition, if your baby shows any of these signs of water intoxication, head to the hospital immediately:

  • inconsolable crying
  • vomiting
  • lethargy
  • seizures
  • tremors

Fortunately, parents are usually aware — by word of mouth or from their pediatrician — that they shouldn’t give young babies water. But now you also know the why behind the guideline.