For babies younger than six months, the answer is generally no; their primary source of nutrition and hydration must be breast milk or formula. Introducing plain water too early can pose health risks and is not recommended by major health organizations. Understanding the composition of milk and the physiology of an infant explains why this guideline exists for the first half-year of life.
Primary Reasons to Avoid Water for Infants
Giving plain water to an infant under six months old carries two major health risks related to their small stomach capacity and immature physiology. The first concern is nutrient dilution. Water fills the baby’s stomach without providing necessary calories or nutrients, replacing the volume that should be occupied by nutrient-dense milk or formula. This leads to a reduction in the intake of fat, protein, and carbohydrates necessary for rapid growth and development, negatively impacting weight gain.
The second, more serious risk is water intoxication, medically known as hyponatremia, which results from a dilution of the body’s sodium levels. Excess water overwhelms the baby’s kidneys, which are not yet mature enough to filter large volumes efficiently. When the sodium concentration in the bloodstream drops too low, cells throughout the body swell as water moves in to balance the concentration. This swelling is particularly dangerous in the brain, where the confined space of the skull can lead to serious complications. Symptoms of hyponatremia can include irritability, drowsiness, hypothermia, and potentially seizures or brain damage.
Hydration Sources in Breast Milk and Formula
Infants do not require additional fluids because their milk source is perfectly suited to meet all their hydration needs. Mature breast milk, for example, is composed of approximately 87% to 88% water. This high water content provides sufficient fluid for all necessary functions, including regulating body temperature and aiding digestion.
Correctly prepared infant formula similarly contains a balanced proportion of water and solids designed to mimic the hydration properties of breast milk. This measured hydration is ideal because the infant kidney is naturally immature and cannot efficiently handle the large solute load or rapid fluid shifts that plain water introduces. Parents can monitor their baby’s hydration status by checking for frequent wet diapers, which should number at least four to six good ones every 24 hours. This confirms that the milk or formula is providing everything the baby needs.
When to Safely Introduce Plain Water
Water can be introduced around the six-month milestone, coinciding with the introduction of solid foods. At this age, the digestive system and kidneys have matured slightly, and the baby’s diet begins to diversify. Offering water at this stage serves a different purpose than hydration, which is still primarily supplied by breast milk or formula.
The initial introduction helps the baby learn the motor skills involved in drinking from an open cup or straw cup, supporting developmental progress. A few sips of water with a meal can also help rinse the mouth after eating solid foods. Parents should start with very small amounts, typically limiting the daily intake to less than 2 to 4 ounces to prevent displacing valuable nutrition. Milk or formula must remain the main source of nutrition and hydration until at least the first birthday.