Final answer:
Infants have a high percentage of body water that composes nearly half of their weight at birth, which is about 75%. This water composition decreases to approximately 65% by 4 months of age, still accounting for around half of the infant's weight.
Step-by-step explanation:
The question addresses what composes half of the weight of an infant before and at 4 months of age. According to comprehensive growth data, a newborn's weight is mainly composed of water. At birth, infants have a high percentage of body water—around 75%. This body water contributes to half of the infant's weight. As the infant grows, the water composition decreases to about 65% by the age of 4 months.
In the first few months after birth, infants typically lose a small amount of weight, which is a result of the resorption and urination of the fluid that fills their lungs initially. However, after this initial weight loss, infants begin to gain weight rapidly. It is expected for infants to double their birth weight by around 6 months of age and to triple it by their first birthday.
The substantial water content in infants plays a crucial role in their overall weight during the early months of life. This is why water accounts for approximately half of an infant's weight before and at 4 months.