Get informed on baby growth averages for every month, but keep in mind: Whether your little one is in the top or bottom percentile, your baby is probably doing just fine.

By Petronio C. Yadao, MD
Sara Nicole Garavuso/Shutterstock

While newborns vary in size and shape as much as adults do, full-term babies tend, on average, to weigh between 5 pounds, 11 ounces and 8 pounds, 6 ounces. They're usually between 19 and 21 inches long, with a head circumference of about 13 1/2 inches.

Your baby is more likely to be on the larger side at birth if:

  • You and your partner are larger than average
  • Your baby is a boy
  • You developed gestational diabetes during pregnancy
  • Your baby was overdue

Your baby is more likely to be on the smaller side at birth if:

  • You and your partner are smaller than average
  • Your baby is a girl
  • You suffered from high blood pressure or asthma during pregnancy
  • You had poor nutrition habits during pregnancy
  • You smoked, drank, or used drugs during pregnancy
  • You gave birth to multiples

As you can see, there are many contributors to birth size. Thus, it's not possible to accurately predict childhood or adult size from how big or small your baby was at birth. 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. This is normal. Babies are born with extra body fluid that they lose over the first five days. They usually regain this weight during the following five days, so that by about day 10, they should be back to their original birth weight. Form that point on, most infants experience rapid growth.

Charting Baby's Growth

Growth charts are an easy and accurate way to track how your child is growing. At each checkup, your doctor will measure your child's height, weight, and — if he's younger than 2 — head size and plot the results on a chart to compare him 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 percent of boys his age and weighs more than half of them do.

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 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."

Here's a snapshot of infant height and weight averages by month based on the WHO growth charts to give you an idea of how growth is tracked:

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 pediatrician will measure the distance around your baby's head. This measurement, known as the head circumference, is used to gauge baby's brain growth.

A baby's skull, and consequently her head, needs to increase in size to accommodate the growing brain. The soft spots on your baby's head, called fontanels, remain open in the first 18 months to allow for this expansion. In general, a newborn's head measures about 13 1/2 inches. Your baby will experience rapid head growth, particularly during the first four months, and by the end of the first year, her head circumference will be about 18 inches. She will add another inch in the second year, bringing her head close to the adult size.

Average Doesn't Mean "Normal"

Any number between the fifth percentile and the 95th is considered "normal." So don't get hung up on 50th percentile averages.

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

Not to mention, there are lots of factors that affect the growth of healthy growing babies. For example, whether you decide to breast- or bottlefeed will influence your baby's rate of growth in the beginning. Exclusively breastfed babies typically gain weight faster in the first two months of life compared with 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- or formula-feeding has a long-term effect on a child's growth.

Detecting a Problem

If your pediatrician is concerned about a change in your baby's growth pattern, she will ask you about your child's feeding habits and how many calories she takes in a day to make sure that your child is eating enough. In addition, your doctor will ask if your baby is hitting her developmental milestones, and about any recent illnesses and other behavior and social conditions. She will examine the baby, looking for a sign of a physical problem that may be a clue to the cause of, say, a large drop in baby's weight percentile.

Based on the checkup, the physician may decide to do blood or urine tests, which can detect problems including low blood count, diabetes, and kidney disease — all of which may affect a child's growth.

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

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Comments (1)

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