Final answer:
Infants typically gain an average of 0.45 kg (1 lb) per month between three to six months of age, leading to a doubling of their birth weight by the six-month mark.
Step-by-step explanation:
The average weight gain for an infant between three to six months of age is generally around 0.45 kg (1 lb) per month. This slower pace follows the initial rapid weight gain of about 28 g (1 oz) per day during the first two months. By following this rate, it is expected that an infant will approximately double their birth weight by six months after birth.