Most babies weigh between 5 1/2 and 10 pounds. The average length of a newborn is 20 inches (50 centimeters), with a range of 18 to 22 inches. The average head circumference (the distance around the head) is 14 inches (35 centimeters).At each visit, your doctor charts your child's growth.f you have an average-size baby, you can expect her to double her birth weight by five months of age, triple it by one year, and quadruple it by two years. The average gain in length or height is 10 to 12 inches in the first year and 5 inches in the second year.