A full-term newborn is about 75 to 78% water by body weight. That’s significantly higher than an adult, who averages around 50 to 60% water depending on age and sex. Babies are, quite literally, among the most water-dense humans on the planet.
How Water Content Changes Before Birth
A baby’s water percentage actually peaks long before delivery. During the first trimester of pregnancy, a developing fetus is roughly 94% water. By 24 weeks of gestation, that number drops to about 86%. At full term (around 40 weeks), it settles to approximately 78%. This steady decline happens because the fetus is building fat, muscle, and bone tissue that gradually replace some of that water content.
Premature babies reflect this timeline. An infant born between 26 and 31 weeks of gestation is typically 80 to 85% water, while very small preterm babies (under about 3.3 pounds) measure around 83%. The earlier a baby is born, the higher its water percentage.
Why Babies Carry So Much More Water Than Adults
The difference between a newborn at 75 to 78% water and an adult man at 59% (or an adult woman at 50%) comes down to body composition and how water is distributed. In adults, about two-thirds of body water sits inside cells and one-third sits outside them, in blood plasma, between tissues, and in organs. Newborns have the opposite ratio: the fluid outside their cells is about 1.5 times the volume of the fluid inside them.
This flips during the first few years of life. By age 3, children reach the adult pattern, with twice as much water inside cells as outside them. That ratio holds steady for the rest of life. The extracellular compartment, which makes up about 40% of a newborn’s body weight, shrinks to roughly 20% by age 10.
Adults also carry more body fat than babies do in the first weeks of life, and fat tissue contains very little water. As people age and typically gain fat while losing lean mass, their overall water percentage drops further. Men over 51 average about 56% water, while women in the same age group average 47%.
How Quickly the Percentage Drops After Birth
In the first few days after birth, a healthy newborn loses water rapidly. This is why it’s normal for babies to lose 5 to 10% of their birth weight in the first week. Most of that early weight loss is water, particularly from the large extracellular compartment that the baby no longer needs outside the womb. The kidneys begin filtering more efficiently, and excess fluid is shed through urine and skin.
Over the following months, the total body water percentage continues a gradual decline as the baby builds fat stores, muscle tissue, and denser bones. By the time a child reaches school age, their body water percentage is much closer to adult levels.
Why High Water Content Makes Dehydration Dangerous
Because so much of an infant’s body is water, and because so much of that water sits outside cells where it can be lost quickly, babies are far more vulnerable to dehydration than adults. A baby with mild dehydration has already lost about 3 to 5% of their body weight in fluid. Moderate dehydration, where the heart rate starts to climb, represents a 6 to 8% loss. Severe dehydration, at 10% or more, can compromise blood pressure and organ function.
These thresholds hit faster in a small body. A 10-pound baby losing 5% of body weight has shed just 8 ounces of fluid. Vomiting, diarrhea, or fever can push an infant to that point within hours rather than days.
Why Babies Shouldn’t Drink Plain Water
Given that babies are already so water-rich, it might seem like they’d need extra water to maintain those levels. The opposite is true. Breast milk and formula already contain all the water a baby needs for growth and to replace what’s lost through breathing, sweating, and digestion.
Giving plain water to an infant under 6 months is genuinely dangerous. Their kidneys are too immature to handle a sudden influx of water with no electrolytes in it. The result can be water intoxication: sodium levels in the blood drop rapidly, cells in the brain swell, and the baby can develop seizures, abnormally low body temperature, or altered consciousness. A total body water increase of just 7 to 8% from excess plain water is enough to trigger these symptoms. For a small infant, that’s a surprisingly small volume of water.
The CDC has specifically warned against supplementing with tap or bottled water for babies under 6 months. Even for formula-fed infants in hot weather, the formula itself provides adequate hydration in nearly all cases.