Final answer:
Babies typically lose three to seven percent of their birth weight in the first week due to shedding of initial fluid and feeding establishment. They then gain about 20 g (0.7 oz) per day after the first week, double their weight by six months, and by their first birthday, they usually triple it.
Step-by-step explanation:
It is a common occurrence for babies to lose between three to seven percent of their birth weight in the first week of life. A baby with an average birth weight of 3.4 kg (7.5 lb.) may see their weight decrease to about 3.2 kg (7.1 lb.) during this period. This decline is largely due to the shedding of excess fluid present at birth, as well as the time it might take for feeding to become established. Post the initial week, a healthy infant is expected to gain approximately 20 g (0.7 oz) per day.
After the initial weight reduction, normal weight gain is roughly 1 oz (28 g) per day for the first two months, slowing thereafter to about 1 lb. (0.45 kg) per month for the balance of the first year. By the age of six months, an infant's weight generally doubles, and by the first birthday, it typically triples. The rate of growth, in terms of body length, is especially notable during the first half of infancy, with averages being 2.5 cm (1.0 in.) per month for the first six months and 1.2 cm (0.5 in.) per month for the latter six.