Many new mothers tend to compare their babies with the average to make sure they're developing properly. Babies grow an average of 10 inches in length and can triple their birth weight in the first year of life, but not all of them grow at the same rate, according to the website, Kids Health. Doctors' growth charts can show you the average, but just because your baby may fall outside that average doesn't mean that something's wrong.
Babies grow fastest during the first six months, averaging 1/2 to 1 inch per month, and 5 to 7 oz. per week. But this slows gradually as the baby gets older to about 3/8 inch per month and 3 to 5 oz. per week from 6 months to 1 year. By the time your baby is a year old, he should be about double his birth height and triple his birth weight.
Average 9-month-old Boy
According to the World Health Organization growth charts for children under 2, the average 9-month-old boy weighs about 20 lbs. and is about 28.5 inches long with a 17.5-inch head circumference. The lowest percentile, which would indicate that a small baby boy is at 16 lbs. and 26 inches with a 16.75-inch head circumference. The top percentile, which would indicate a large baby boy, is at 25 lbs. and 30 inches, with an 18.5-inch head circumference.
Average 9-month-old Girl
The WHO chart indicates that an average 9-month-old baby girl weighs about 18 lbs. and is about 27.5 inches long. The lowest percentile for that age is 15 lbs. and 25.5 inches, and the highest is about 23.5 lbs. and 29.5 inches. The average head circumference is about 17.25 inches, with the lowest percentile being about 16 inches and the largest about 18.25 inches.
Outside the Norm
If your baby doesn't match up to the average, it's not necessarily a cause for worry. The standards were set using the growth patterns of breastfed babies, so a formula-fed baby may show slower growth. Babies can grow in spurts, too, notes Kids Health. Your baby may be in a low percentile today but average next month or above average the month after that. Talk to your child's pediatrician if you have any major concerns, and he'll likely monitor her growth long-term rather than use her current percentile as a definite symptom of a developmental problem.