Giving plain water to a baby under six months of age carries significant health risks and is strongly discouraged by medical professionals worldwide. Before this age, an infant’s body is not equipped to process the fluid like an adult’s, and introducing water can quickly lead to dangerous internal imbalances. The hydration and nutritional needs of a young infant are perfectly met by breast milk or formula alone, and offering any other liquid can interfere with their development.
The Physiological Reasons Water is Harmful to Infants
A fundamental reason water is restricted for young infants lies in the immaturity of their kidneys. Unlike adult kidneys, a baby’s renal system is still developing and cannot handle a large volume of solute-free water. Introducing plain water forces these immature kidneys to filter a load they are not ready for, leading to the rapid loss of essential substances from the bloodstream.
This process directly affects the delicate balance of electrolytes within the body. Electrolytes, especially sodium, are charged minerals required for nerve signaling, muscle function, and maintaining fluid levels inside and outside cells. When a baby consumes too much water, the sodium concentration in their blood becomes diluted, a condition called hyponatremia. Even small amounts of water can disproportionately affect this balance because an infant’s body composition is about 75% water.
Beyond the physiological processing issues, water poses a significant threat through nutritional displacement. A newborn’s stomach capacity is incredibly small. Breast milk and formula are already composed of approximately 80–90% water, providing all the necessary hydration alongside concentrated calories and nutrients. When a baby’s tiny stomach is filled with plain water, it quickly creates a feeling of fullness.
This premature satiety prevents the infant from consuming the necessary volume of breast milk or formula, which are the only sources of nutrition they need to thrive. The resulting calorie deficit can interfere with their growth and development, which is why exclusive milk feeding is recommended. This risk of malnutrition is compounded when parents over-dilute formula with extra water, reducing the caloric density of every feeding.
Understanding Water Intoxication in Babies
The medical condition resulting from excessive water intake is known as water intoxication, the central feature of which is hyponatremia. Hyponatremia means “low sodium in the blood,” occurring when the concentration of sodium drops below 135 milliequivalents per liter (mEq/L). This dilution causes the body’s cells to absorb the excess water, leading to cell swelling.
In mild to moderate cases of water intoxication, the symptoms may be subtle. Parents might observe irritability, unusual lethargy, or excessive drowsiness. Other signs include vomiting, nausea, and a noticeable puffiness or swelling of the face and extremities (edema). A particularly concerning sign is a drop in body temperature, or hypothermia, sometimes falling below 97°F (36°C).
The most serious danger arises when cellular swelling affects the brain, causing cerebral edema. The skull is a fixed space, and the brain has no room to expand, leading to a dangerous increase in intracranial pressure. This pressure can quickly result in severe symptoms, including seizures, loss of consciousness, brain damage, or death. If a baby exhibits severe symptoms like a seizure or unresponsiveness, immediate emergency medical attention is required.
Official Guidelines for Introducing Water
For the first six months of life, health organizations strongly recommend that infants receive only breast milk or infant formula. This guideline applies universally, even in hot climates or when an infant has a fever, because milk provides perfectly balanced hydration. The appropriate response is to offer more frequent feedings of milk, not plain water.
Once a baby reaches approximately six months of age and begins consuming solid foods, water can be introduced in small amounts. At this stage, water is offered primarily to help them practice cup drinking and aid in the digestion of solids, not as a main source of hydration. Water must remain a supplement, as formula or breast milk should continue to be the primary beverage until at least 12 months of age.
The amount of water should be limited to prevent nutritional displacement, with experts suggesting no more than two to four ounces per day, generally offered during mealtimes. Adding water to formula to stretch the supply is dangerous because it dilutes the necessary nutrients and increases the risk of water intoxication. Following these established guidelines ensures the baby receives optimal nutrition while avoiding health risks.