What the heck do percentiles mean?

Percentiles are a clinical measure your pediatrician uses to plot your child’s overall physical growth on a chart of the general population. So after a baby’s height, weight, and head circumference are measured at a well-baby check-up, your pediatrician plugs that data into a nifty chart that then generates your baby’s percentile ranking.

What does the ranking actually mean? If Baby is ranked 65th percentile for weight that means out of 100 babies in the general population, he weighs the same or more than 65 of them and less than 35 of them.

Most pediatricians use a growth chart created by the World Health Organization (WHO) for kids under age 2 because WHO standards are based on really high-quality global research. The current chart uses data released in 2006 and includes measures from children in 6 different countries, including the U.S., where the environments were believed to support optimal growth. It’s worth noting that this is a huge improvement over past charts, like one widely used in the 1980s that was based on data from one study of white, middle income, formula-fed kids from a small part of Ohio. Not exactly a representative sample.

As any decent pediatrician will tell you, what matters most is baby’s growth over time. The main reason your doc probably even uses the percentile chart is to make sure your baby’s growth is steady.  If for 9 months he was around 80th percentile for weight and then suddenly at one check-up he dropped to the 40th percentile, your doc will probably want to look into why this may have happened.

A doctor explained it really well to my friend when she said, “If your baby’s gotten on the 20th percentile bus, I expect her to stay on it, but if she comes into her next appointment on the 80th percentile bus, I want to know why she transferred.”

There is also no magic number you need your baby to hit for your doc to emphatically yell, “Winner, winner, chicken dinner!” There are healthy babies on both ends of the growth chart, thanks to genetics. For example, if a baby is 95th percentile for height and his parents are both professional basketball players then the pediatrician probably won’t sweat it. My friend was freaking out because her son was in the 8th percentile for weight, but both her and her husband are slender people so it didn’t worry her doctor at all.

Even though everyone knows that some babies/kids/adults are big and some are small, you’ve probably heard new moms at the playground or the library clucking about percentiles with either great pride (“Johnny is in the 90th percentile for height right now!) or with fear and worry (“Jane is only in the 15th percentile for weight this month.”) but these percentiles don’t necessarily predict height and size outcomes either.  I’m living proof of that. I was a giant baby (10lb4oz, sorry mom ) – they actually had to go up to Pediatrics in the hospital I was born in to get diapers for me because I was too big for the newborn size they had. Then I grew into the absolute smallest, skinniest kid on the planet – I was always the one the photographer grabbed for the school photo as his starting point. Now, I’m just average, I’m sort of short at 5’4” but nothing extreme, well, except for my large personality and grand wit of course  ::snort::

It’s the first time your baby has ever been officially ranked next to his peers so it’s easy to get a little batshit crazy, but please know there are other measures that you are kicking ass, that have nothing to do with whether your baby has a particularly big head or is a bit of a string bean.

Hey, somebody has to be the biggest kid, somebody has to be the smallest, and somebody has to be in the middle. It may as well be your kid, right?

More from Amy Morrison

Round Up – Issue 12 January 2014

Aw poop, I missed December’s Round Up. I will forever underestimate how...
Read More


  • I clicked from a link on Pinterest to see how you’d explain it, even though it wasn’t really news to me. The main reason I’m commenting is because I wanted you to know that I kept reading because it was so delightfully written.

  • My son is in the 90th percentile for height and the 50th for weight and has been since birth. The doctor says even though he’s skinny, he’s still on the same curve. The doctor looks at my husband and I ( who are about 6′ tall and thin) and tells us she’d be more concerned if he didn’t look like us.

  • Although a lot of your information is someway correct you have to use the height, weight and head curcumference together – otherwise the data is pretty meaningless! Telling someone their child is on the 90th centile for height tells you nothing… If they are then on the 25th centile for height and head circumference they are very overweight! Or like the post above if the are on the same centile for height & weight but their head circumference is rapidly increasing centiles there may be a medical problem…

  • Thank you for sharing this. I’m a new mom of a 4 month old. When I go to the pediatrician and she hands me these charts I just smile and nod as she explains, but I’m still clueless as to what it really means. This helped.

  • Great post! We too find parents worry if their baby is at the lower end of the percentiles. If the growth is consistent we suggest they look around. Are all adults the same size? We like your friend’s analogy of staying on the same bus!

  • SO glad you shared this! Many mom’s (especially first timers) put so much emphasis on the charts, and it just never gives you the whole picture of your child’s health. It may also be important to mention that a breastfeeding mom should ask for a growth chart for breastfed babies (their curve is usually shaped differently), it’s different than the one you mentioned, which though based on a larger group, is still for formula fed babies (I may be wrong though).

  • You may want to add a little tidbit about head circumference growth as well, since growing too fast can mean hydrocephalus.

    My son was born with a 98th percentile head circumference (can we all just take a moment to THANK GOD for C-SECTIONS!), and his head started growing too fast at 12 months. He has been evaluated by a neurologist and is all good 🙂

    You’re totally right that it’s all about staying on the same curve. His big head wasn’t the problem. The growth was the problem.

Leave a Reply

Your email address will not be published. Required fields are marked *