Final answer:
A baby should regain its birth weight by two weeks, double it by six months, and triple it by one year. The initial weight decrease after birth is due to diuresis of extravascular lung fluid and delayed feeding establishment.
Step-by-step explanation:
After birth, neonates typically experience an initial weight decrease before starting to gain weight. A baby should regain birth weight by approximately two weeks, achieve a weight that is double the birth weight by six months, and triple the birth weight by the end of the first year. The weight loss observed in the first week is due to diuresis of extravascular fluid, including the resorption and urination of fluid that fills the lungs, as well as a delay in the establishment of feeding.
The rapid growth in both weight and length during infancy aligns with these milestones, with an average gain of 28 g (1 oz) per day during the first two months, then slowing to around 0.45 kg (1 lb) per month afterwards. Length also increases, with infants growing about 2.5 cm (1.0 in.) per month in the first half of the year and 1.2 cm (0.5 in.) per month in the second half. These guidelines reflect average growth patterns although individual development can vary.