The Dubious System for Measuring the Size of Your Baby’s Head

Why so many parents are mistakenly told that their kid has a massive noggin.

An infant with a tape measure in the background as if to measure its head.
Photo illustration by Slate. Photos by iStock.

My daughter Mary was a healthy little girl at birth: 7.8 pounds and slightly on the tall side. But her head circumference was almost off the charts. At 14.2 inches around, her skull was in the 96th percentile of all newborns. “She has a huge brain!” I crowed to anyone who would listen. “She’s going to be brilliant!” I was joking, of course, but I was also secretly a little proud of the kid’s massive noggin.

Doctors measure a baby’s head at every appointment in the first few years of life, and there are a lot of appointments. Mary bobbed around a bit on the charts as she grew, but her head stayed impressively ample. At her most recent appointment, her doctor printed out a chart from the World Health Organization that showed my now 2-year-old’s head was still bigger than 93 out of 100 babies her age.

By that time, however, I’d started to suspect that she was not actually the giant-skulled wonder I’d been led to believe. A curious number of my close friends with new babies seemed to be bragging about the same distinction, for one. “His head is enormous!” one set of houseguests told us, showing off their adorably chunky infant. A few weeks later, another old friend stopped by for brunch and told us his year-old son’s head was as big as they come. You only need to browse a few parenting message boards to see that the internet is teeming with parents exclaiming over their kids’ prodigious skulls. When I asked the parents on Slate Slack about their babies, several people immediately piped up: A 1-year-old at the 97th percentile, a 2-month-old at the 96th. One editor has a toddler whose head is at the 96th percentile and body weight at the 18th. It was starting to feel like I lived in a bobblehead version of Lake Wobegon, where all the children’s heads were above average.

But it turns out that this phenomenon is actually the result of some general confusion about what constitutes an oversized baby head. When Carrie Daymont, an assistant professor of pediatrics at Penn State, was a resident in medical school, she found herself having conversation after conversation with parents whose infants’ heads measured as abnormally large or fast-growing. In the clinical context, heads at the extreme upper end of the growth curve are cause for legitimate concern, not cheeky gloating. She wondered: Were this many babies at risk for problems like brain tumors or cysts, for which the measurement for macrocephaly—an abnormally large head—is meant to screen? Or were the charts themselves inaccurate?

What Daymont found when she started looking into this will crush the pride of any parent who relishes bragging about their offspring’s colossal cranium. She started with a data set that included the head measurements of 75,000 pediatric patients spanning three states, and compared those measurements to the WHO chart. If the WHO chart is accurate, then 5 percent of babies should be above the 95th percentile, 10 percent above the 90th percentile, and so on. That’s not the case. From birth to age 2, fully 14 percent of babies were above the 95th percentile, according to the WHO’s chart.

By age 2, 18 percent of children were above that cutoff—which means it’s not really the 95th percentile, but the 82nd. “We’re talking about small differences in head size,” Daymont told me. But the 97th percentile on the WHO chart, I very much doubt that’s the 97th percentile of babies in the United States.” Her findings were published in the journal Pediatrics in 2010. And she’s not the only one to persuasively question the accuracy of the way we assess head size. One 2013 study, for example, suggested that a previously suspected connection between autism and fast-growing brain size may instead reflect inaccurate head-circumference standards, rather than an actual pattern of atypical growth in children with autism.

The WHO curve, published in 2006, is based on measurements of children in six countries, including the United States. The Centers for Disease Control and Prevention started recommending that pediatricians use the WHO curve to assess children from birth to age 2 in 2010, and it is now the widely accepted standard among pediatricians in the United States. The CDC also maintains its own curves. But those curves, too, failed to reflect the actual baby heads Daymont surveyed: 12 percent of 1-year-olds measured above the 95th percentile, for example. The CDC’s curve is also steeper than the WHO’s, which means it’s likelier that a kid will look like his head size is fluctuating in size between appointments, which could prompt even more unnecessary worry and medical treatment.

How could this chart be so askew? The WHO chart’s premise is that all healthy, breast-fed, economically stable children grow in basically the same way, no matter who they are or where they are born. But the one-head-size-fits-all approach doesn’t reflect reality. Head size seems to vary slightly between populations, possibly due to genetic or epigenetic variations. Researchers who compared the WHO curve to actual head measurements in 55 countries and ethnic groups in 2014, for example, concluded that the use of a single international standard for assessing head circumference simply doesn’t make sense. And American babies seem to be far enough off the international standard that Daymont believes the WHO curve may not be the right one for babies born in the United States.

If this line of inquiry carries an uncomfortable whiff of eugenics for you, fear not: There are some studies that show very weak relationships between intelligence and both head circumference and height, but we cannot directly infer anything meaningful about an individual’s or ethnic group’s intelligence based on those measurements. The reason doctors wrap that little measuring tape around babies’ skulls millions of times a year is not to give parents a fun stat to jokingly brag about, but to screen for serious abnormalities. So in this age of widespread parental obsession with the size of their babies’ domes, it’s amazing how few of us know what “normal” really means.