How to Understand Infant Growth Charts

Growth charts are an easy and accurate way to track your child's growth. Health care providers use World Health Organization (WHO) growth charts to monitor growth in infants through age 2.

At each checkup, a health care provider will measure your child's height, weight, and—if they're younger than 2—head size. They will then plot the results on the WHO growth chart to compare them with other kids the same age and sex. So if your 2-year-old son is in the 75th percentile for height and 50th percentile for weight, that means he is taller than 75% of males his age and weighs the same as average males his age.

Don't stress about your child's specific percentile: "It doesn't really indicate how well she's growing," says Richard Ball, M.D., a retired pediatrician at Akron Children's Hospital in Akron, Ohio. "The key is whether she is consistently at the same percentile. If she slows down in length, for example, or shoots up in weight, we're more likely to be concerned."

Read on to learn about birth weight and typical growth in infants.

Size Variations in Infants

Newborns vary in size and shape as much as adults do. According to the 2020 National Vital Statistics Reports, full-term babies tend, on average, to weigh between 3,000 and 3,499 grams (6 pounds, 9 ounces, and 7 pounds, 11 ounces). In addition, they're usually between 19 and 21 inches long, with a head circumference of about 13 1/2 inches.

There are many contributors to a child's size and growth. Thus, it's impossible to accurately predict childhood or adult size from how big or small your baby was at birth. For example, a tiny newborn won't necessarily be a petite adult, and a large baby isn't guaranteed to grow up to be a football player.

Many parents become concerned when their baby loses weight in the first few days of life, but rest assured that some weight loss after birth is normal. Babies usually regain this weight during the following five days, so by about day 10, most are back to their original birth weight.

According to the American Academy of Pediatrics (AAP), 50% of newborns are at or above their birth weight on days 9 and 10. From that point on, most infants experience rapid growth. Only 14–24% are not yet back to birth weight by day 14, and 5-8% remain below birth weight at 21 days.

Factors That Influence Birth Weight

While you can't always predict a baby's weight prior to birth, according to the National Library of Medicine, there are some known factors that can increase the likelihood that your baby will have a high or low birth weight.

Your baby is more likely to have a high birth weight (more than 8 pounds, 13 ounces) if:

• You and your partner are larger than average
• You have type 1 or type 2 diabetes, or developed gestational diabetes during pregnancy
• Certain genetic conditions
• You gained excess weight during pregnancy

Your baby is more likely to have a low birth weight (less than 5 pounds, 8 ounces) if:

• You experienced health problems or infections during pregnancy
• You took certain medications during pregnancy
• You used substances or smoked during pregnancy
• You are pregnant with multiples
• You had problems with your placenta
• Certain genetic conditions or congenital disabilities

Charting Baby's Growth

Here's a snapshot of infant height and weight averages by month based on the WHO growth charts.

50th percentile growth for boys

50th percentile growth for girls

Measuring head circumference

In addition to measuring weight and height at well-baby visits, your health care provider will measure the distance around your baby's head. This measurement, known as the head circumference, is used to gauge a baby's brain growth.

A baby's skull, and consequently their head, needs to increase in size to accommodate their growing brain. The soft spots on your baby's head, called fontanels, remain open in the first 18 months to allow for this expansion. On average, a newborn's head measures about 13.5 to 14 inches in circumference, according to the Centers for Disease Control and Prevention (CDC).

Your baby will experience rapid head growth, particularly during the first four months, and by the end of the first year, their head circumference will be about 18 inches. They will add about another inch in the second year.

Average Doesn't Mean "Normal"

Many parents worry that their baby may not be gaining enough weight. However, there is a wide range of "normal" on growth charts, so don't get hung up on 50th-percentile averages.

It's important to remember that one measurement neither predicts how your baby will grow nor does it necessarily suggest a growth problem. As long as your baby grows steadily and consistently, there's usually nothing to worry about.

Not to mention, there are lots of factors that affect the growth of healthy babies. For example, whether you decide to breast- or bottlefeed will influence your baby's growth rate in the beginning.

According to the CDC, exclusively breastfed babies typically gain weight faster in the first two months of life than formula-fed babies. But these infants also tend to grow less rapidly than their formula-fed peers through about 1 year. Ultimately, however, neither breast- nor formula-feeding has a long-term effect on a child's growth.

Using Growth Charts to Detect a Problem

If a health care provider is concerned about a change in your baby's growth pattern, there are some things they will do to evaluate the situation. According to Boston Children's Hospital, diagnosing a growth problem in children may include:

• Observing your child's growth over time
• Completing a medical history and physical exam
• Inquiring about your baby's developmental milestones
• Considering the size of a child's relatives
• Blood tests to evaluate hormones and chromosomes, and rule out conditions linked to growth disorders
• Scan of the pituitary gland to detect growth hormone deficiency
• X-ray of hand or wrist to evaluate bone size relative to your child's age

The growth and development of your baby is an area of great excitement and concern for parents. Keeping informed, taking your baby to their well-baby visits, and working closely with your pediatrician are the first steps to walking assuredly through these extraordinary times of change.