Your Baby’s Head Is Probably Not Gigantic 

In those early days of new parenthood, a trip to the doctor’s office for a baby check-up can feel like a grand occasion. Aside from giving you a reason to put on pants and interact with real-life adults, it’s an opportunity to finally get some quantifiable data on this tiny being whose existence of eating, sleeping and pooing can seem so erratic. The big test is when the nurse takes your baby’s measurements.

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“How’s my little one growing?” mums and dads ask in anticipation. The numbers they hear might leave them surprised, amused and sometimes concerned.

“She has a giant head,” my sister tells me when giving a weekly update on her new baby. “Ninetieth percentile.”

“Yes!” my dad says. “She’s going to be a genius!”

Let’s not crush the dreams here, but the conversation seems to be a common one. On Facebook, I often see parents reporting the results of their baby’s latest check. A theme: average weight, enormous heads. Ninety-fifth percentile. Ninety-ninth percentile.

A number that’s off the charts. A relative tells me that from the time her son was to born to his one-month checkup, his head “skyrocketed from 66 per cent to 98 per cent.” (My own child did not have a particularly noteworthy infant head size — hers was around 60 per cent.)

Slate writer Ruth Graham noticed that curious number of her friends seemed to bragging about their babies’ large or fast-growing noggins. “It was starting to feel like I lived in a bobblehead version of Lake Wobegon, where all the children’s heads were above average,” she writes. So she started digging.

What she discovered was that the phenomenon has to do with inaccurate head-circumference standards.

At least in the US, babies born are examined based on a curve published by the World Health Organisation. Graham cites the research of Carrie Daymont, an assistant professor of pediatrics at Penn State, who found that the curve fails to reflect the actual heads of babies in the population:

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 per cent of babies should be above the 95^th percentile, 10 per cent above the 90^th percentile, and so on. That’s not the case. From birth to age 2, fully 14 per cent of babies were above the 95^thpercentile, according to the WHO’s chart.

By age 2, 18 per cent of children were above that cutoff — which means it’s not really the 95^thpercentile, but the 82^nd.

The inaccuracies have to do with the fact that we’re using a one-size-fits-all chart that’s based on measurements of babies in several countries — when in fact, as Graham explains,”head size seems to vary slightly between populations, possibly due to genetic or epigenetic variations.” To make matters more confusing, doctors plot each measurement on a different growth chart: one from the Centres for Disease Control and Prevention (CDC).

Implications of this, of course, go beyond seeing a disproportionate number of parents talk about their babies’ potentially gigantic brains on Facebook. A purpose of the of head-circumference growth curves is to identify children who may need extra testing because of a large head, called macrocephaly, or a rapidly growing head.

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Daymont found that when using the CDC curves, the percentage of children diagnosed with macrocephaly changes with age. “One-fifth of the expected number of one-month-olds and 2.5 times the expected number of 1-year-olds were classified as having macrocephaly, or being above the 95th percentile,” a report states. “Many of these older infants above the 95th percentile may have received unnecessary follow-up tests and specialty referrals, and their parents may have been unnecessarily worried.”

Head circumference measurements are important — a friend Heather tells me that having her son’s head measured at his one year appointment was a “saving grace” in detecting hydrocephalus, a condition that affects one to two of every 1000 babies according to some estimates. “If not for that routine measurement, it would have been really bad,” she says.

What we need, though, is a better measuring system.