According to healthy weight ranges compiled by the CDC, infant boys at 11 months who weigh around 21 lbs. on track with about 50 percent of their peers. For females, this number is approximately 19 lbs. Boys average 7 lbs. at birth, so the growth rate is about 1.2 lbs. per month. Female infants average a growth rate of about 1 lb. per month. Eleven-month-old boys who measure approximately 29 inches in length are equal to 50 percent of their peers. According to the CDC charts, newborn boys average 20 inches in length, so the average growth is approximately .82 inches per month.