Average Baby Length in the First Year

Whenever one of my friends has a baby, I typically get a celebratory text that includes baby’s name, time of birth, weight and length. Recently, my friend Lori F., mom of two in Creedmoor, North Carolina, asked, “Why does everyone include baby’s length in these messages?”

While infant length may not seem that important to friends and family who’re just eager to see cute newborn photos, it’s a key tool pediatricians use to track baby’s growth. “Clinicians focus on patterns over time, such as if [baby’s] growth is crossing major percentile lines or changing rapidly, rather than on solely one point,” explains Shani A. Jones, MD, MPH, a pediatrician at Bluebird Kids Health in Tamarac, Florida.

So, what’s considered normal infant length—and what factors affect baby’s length? Read ahead for what to know about average baby length—and check out our infant height chart by month.

Key Takeaways

• The average length of a newborn is about 19.6 inches for boys and 19.3 inches for girls.
• Baby’s length is an indicator of the growth and development of their bones, muscles and organs.
• Baby length, which is measured while lying down, is recorded until a child is 2 years old, when the measurement changes to height, which is measured while standing up.
• Percentiles (including length) are a tracking tool for pediatricians to help monitor baby’s growth, including length, over time.
• Genetics, maternal health, feeding and gestational age can contribute to newborn length at birth and baby’s length as they grow.

Average Baby Length of Newborn

The average length of a newborn is around 19.6 inches for boys and 19.3 inches for girls, according to the World Health Organization (WHO). Pediatricians use infant growth charts as a way to monitor baby’s growth. In addition to baby’s length, growth charts also track baby’s weight and head circumference.

Infant Length Chart: Average Growth by Month (0-12 Months)

At each well visit, your pediatrician will measure baby’s length. “A baby’s length is one of the first clues we have about their overall health and development,” says Ilan Shapiro, MD, a pediatrician at AltaMed Health Services in Los Angeles. “From birth through the first year, it helps us see how they’re growing.”

Here’s what to know about normal infant length throughout the first year of life.

Average length for boys (0-12 months)

From the time he’s born until he reaches his first birthday, a baby boy in the 50th percentile will grow from around 19.6 inches to around 29.8 inches, according to the WHO. Here’s a month-by-month breakdown of average baby length for boys.

Average length for girls (0-12 months)

From the time she’s born until she reaches her first birthday, a baby girl in the 50th percentile will grow from around 19.3 inches to around 29.1 inches, according to the WHO. Here’s a month-by-month breakdown of average baby length for girls.

Why Does Baby Length Matter?

Baby’s length is a good indicator of the growth and development of their bones, muscles and organs, says Jenelle Ferry, MD, a board-certified neonatologist at Pediatrix Medical Group in Tampa, Florida. “Growing appropriately for length, and especially maintaining their length and weight ratios, indicates good lean mass growth,” she explains. Ferry adds that an on-track length measurement can indicate that baby’s growing overall, “not just putting on weight from adipose (fatty) tissue.”

Experts emphasize that monitoring growth and development is about looking at trends over a series of measurements, not a single measurement. “A baby’s length might shift slightly from visit to visit due to small things like how they’re positioned [on the measuring board] or who’s doing the measuring,” says Jones. Ferry echoes this and also highlights how difficult it can be to get an accurate measurement of a squirmy baby. So, if baby’s length measurement seems a little bit off at one of their well checks, chances are high that the pediatrician won’t see it as a red flag quite yet.

Length vs. height: What’s the difference?

If you’re confused why babies are measured in length, not height, you’re not alone—I was right there with you before researching this topic. “During the first year, we always talk about length because babies aren’t standing yet,” explains Shapiro. Jones adds that doctors don’t measure height until a child is 2 years old and can consistently stand.

How baby length ties into percentiles

During baby’s checkups, your pediatrician will document which percentiles they fall into when it comes to length, weight and head circumference, which compares them to other babies their age. If baby’s in the 50th percentile, for example, it means about 50 percent of babies measure shorter and the other 50 percent measure longer. If baby falls into the 5th percentile, that means 95 percent of babies measure longer; and if they fall in the 95th percentile, that means only 5 percent of babies measure longer.

If your infant is measuring long or short, it’s not cause for worry, so avoid the temptation to start comparing them to other babies. “Growth and development are so different for every baby, and there are pretty wide ranges of normal, so it’s important to make sure baby is following their path on the growth curve,” says Ferry. “It’s not like passing a test, where 10 percent is failing. Ten percent of all normal babies will be at the 10th percentile. Similarly, having a baby at the 90th percentile doesn’t mean that the child is growing any better (or worse) than the baby at the 10th, 3rd or 20th percentile.”

What Factors Affect the Length of an Infant

There are a number of factors that can contribute to baby’s length from the time they’re born through their first year (and beyond).

Genetics and parental height

As with many things, genetics can play a big role in baby’s length. “You can usually expect that a baby whose parents are both over 6 feet tall will be longer than one whose parents are both 5 feet tall,” says Ferry. Jones echoes this: “Taller parents often have taller babies.”

Gestational age at birth (preterm vs. full-term)

When a baby’s born can also contribute to their size. Babies delivered before 40 weeks of gestation are typically smaller in length, according to the Mayo Clinic.

My daughters were both born pre-term, but their length at delivery is a testament to how much genetics plays into size. My husband (their father) is 6 feet tall, and my youngest measured 19.5 inches long when she was born at just 34 weeks, while my oldest measured 19.25 inches long when she was born at 35 weeks.

Nutrition and feeding

Baby’s feeding and nutrition can also have a significant effect on their length. “A baby who eats well and sleeps regularly tends to grow at a healthy rate,” says Shapiro. “If baby has frequent infections or feeding challenges, growth might slow down temporarily, and that’s something we monitor together.” Jones says that social factors like food security and general access to care can also factor into baby’s growth and measurements.

Maternal health during pregnancy

The birthing parent’s health during pregnancy can also have an effect on a baby’s size at birth. “Maternal nutrition and some complications in pregnancy can affect where baby starts at birth,” says Ferry. Additionally, habits like smoking during pregnancy can have long-term effects on a child’s height.

Normal Infant Length by Month

Normal infant length by month varies widely (remember, 5th percentile is just as normal as 95th percentile!), but based on the WHO growth charts for boys and girls, normal infant length by month is:

When to Talk to Your Doctor About Baby’s Length

Shapiro stresses the importance of remembering that your healthcare provider will review baby’s growth over time, rather than looking at a single measurement. After birth, your pediatrician will measure baby’s length at each well check (there are usually nine during the first year) as a way to collect this growth data. If you’re concerned with a measurement, you can talk to your pediatrician to get their thoughts—and, in many cases, their reassurance.

To Sum It Up

Every baby’s unique, and so is their growth journey. “Some [babies] grow in quick bursts, others more gradually,” says Shapiro. “The key is to celebrate their progress and stay consistent with checkups. With good nutrition, rest and love, every baby has what they need to thrive in that exciting first year of life.”

Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.