A baby should weigh approximately 13 pounds at three months of age. Baby girls weigh between 11.8 and 14.0 pounds at three months old on average. Baby boys weigh between 13.0 and 15.2 pounds, according to Baby Center.
Twenty-five percent of babies weigh more than average, and another 25 percent of babies weigh less than average. Babies gain five to seven ounces a month. Several factors contribute to a baby's weight gain, including his weight at birth, his gestation period and his DNA. Babies born prematurely do not meet the standards set out by physicians for babies born full term.