Many factors affect infant weight gain including illness, metabolism, body shape, feeding methods and personality. Your baby's doctor will likely use a child centile chart for either a baby boy or baby girl from the Royal College of Paediatrics and Child Health to assess your infant's weight.
The first three months
Babies may drop between 5 and 10 percent of original weight after birth. Within two weeks, most infants gain the weight back. At one month, average baby weight is between 2.99 to 5 kilos (6.6 to 11.2 lbs). At three months, normal baby weight ranges between 4.3 and 6.9 kilos (9.5 to 15.4 lbs).
4 to 6 months old
Babies should gain 450 to 900 grams (1 to 2 lbs) monthly the first six months. Weight should range between 4.9 and 7.9 kilos (11 to 17.6 lbs.) at four months and 6.1 and 9.5 kilos (13.6 to 21.1 lbs.) by six months.
9 months old
The Royal College of Paediatrics and Child Health centile chart suggests a range of 7.5 to 10.5 kilos (15.6 to 24 lbs.) for 9-month-old infants. Growth rate averages about 455 grams (1 lb.) per month the first year.
12 months old
By 12 months old, a baby should weigh between 7.9 and 12.1 kilos (17.6 to 26.6 lbs.). Formula-fed babies will have gained, on average, 455 grams (1 lb.) more than their breast-fed peers the first year.
18 months old
An active baby will likely gain at a slower rate than her more laid-back counterpart. By 18 months old, babies should weigh between 9.1 and 13.9 kilos (20 and 30.6 lbs.).
24 months old
By 24 months of age, normal baby weight is between 10 and 15.5 kilos (22 to 34.1 lbs.) and should represent nearly four times his birth weight.