Final answer:
By their first birthday, infants have typically tripled their birth weight. They double their weight by six months and quadruple it by age two.
Step-by-step explanation:
By their first birthday, infants have typically tripled their birth weight. This growth milestone is part of the rapid physical development that occurs during infancy, where a newborn, on average, doubles their birth weight by six months and triples it by twelve months. A well-nourished infant's growth is steady, with a general pattern of gaining weight and length. By age two, the infant's weight will have quadrupled.