In a society full of supermodels and athletic superstars, every parent wants to know how tall their child is going to be. It’s not nearly as mystical or complicated as one might think. There is a simple scientific formula used to calculate a child’s genetic growth potential.

What is “genetic growth potential?”

A child’s genetic growth potential simply means that if you take into account both parents, what is the child’s likelihood of growing to a certain size. It’s quite intuitive. Short parents have short kids. Tall parents have tall kids. If a child is significantly above or below his/her genetic potential, it can signify a problem.

How do I calculate my child’s growth potential? (How tall is my kid going to be?)

Get mom’s height (in centimeters)
Get dad’s height (in centimeters)
Plug it into the simple formula (the trick is converting inches to cm).

FOR BOYS

[Mom’s height + Dad’s height + 13 cm] divided by 2= potential height in cm

FOR GIRLS

[Mom’s height + Dad’s height – 13 cm] divided by 2= potential height in cm

Help with the math

The next question is how to convert to centimeters. It’s simple: Get the total inches and multiple by 2.54 cm (1 inch = 2.54 cm).

EXAMPLES:

Let’s take me, for example. I am 5′ 9″. That is a total of 69″ (5 feet x 12 inches=60 inches, then add the extra 9 inches). Now pull out your calculator: 69 (my total inches) x 2.54 (the conversion factor) = 175.26 cm. I am 175.26 cm tall.
Let’s take my husband, who is 6′ 6.5″. (6′ x 12″ =72″. Then add 6.5″ for a total of 78.5″). 78.5 (total inches) x 2.54 (to convert to cm) is 199.39 cm.
Now put it together in the formula:

My son’s growth potential is calculated like this:

175.26 cm (mom’s height) + 199.39 cm (dad’s height) + 13 = 387.65 cm. Now divide that by 2 and the answer is 193.8 cm growth potential

Great, I figured that my son’s growth potential is 193.8 cm (but since I’m American, I want inches. So work backwards. 193.8 divided by 2.54 (reverse the conversion factor) = 76.3 inches (or 6′ 4.3″) is my son’s growth potential.

My daughter’s growth potential is:

175.26 cm (mom’s height) + 199.39 cm (dad’s height) – 13 = 361.65 cm. Now divide that by 2 = 180.8 cm growth potential (divided by 2.54 to get in inches = 71 inches or 5′ 11″)

Change it up: If I had married someone my same size, swap out 175 cm for the 199 cm in the equations.

Answer: My son’s growth potential would change to 5′ 11″ (not 6′ 4″) and my daughter’s would be 5′ 6″ (not 5′ 11″). So you can see the big difference one parent’s height can mean to a child’s ultimate growth potential.

What if my infant is much bigger/smaller on the growth curves?

It isn’t all that uncommon that I will have an infant that doesn’t match up with the calculated growth potential on the measured growth curves (especially in the beginning). For example, mom is 5′ and dad is 5′ 4″. They have an infant that measures 90% for height and weight. Instinctively, you know that this child should be smaller than average. The key is to remember that how an infant starts out doesn’t always reflect how a child will end long term. The early infancy usually reflects more about the health and growth environment in utero. In this example, the big infant may just be reflecting a perfectly healthy in utero growth and medically we would expect this child to drop into his/her much smaller genetic potential over the first few years of life.

Conversely, a set of tall parents may have a small infant who experienced stress/problems in utero (perhaps delivering premature). When the child is given a hardier growth environment out of the womb, he or she may make big gains in growth and “catch up” to his or her genetic potential.

What if my child is not meeting his or her genetic potential?

If you have just calculated your child’s genetic potential and it is way off (too big or too small), it may indicate a problem. With that said, there are plenty of variations within the same family (e.g., same-gender siblings are not always the same height). If your child is vastly off the expected norm, however, you should discuss it with your pediatrician. A pediatrician is well versed in normal growth patterns and can help you navigate whether the variation is something to worry about.

How can I ensure my child meets his/her growth potential?

While there are no guarantees, there are certainly things you can do to improve your child’s chances.

Ensure a balanced, proper diet. Even with the tallest parents in the world, a malnourished child won’t grow well.
Get good sleep. Getting enough, good sleep is an important part of the healing and restorative processes the body goes through on a daily basis. In order to grow, kids need good sleep.

Other tricks docs use to track growth

While the calculation is a well-accepted scientific calculation, doc’s always have other tricks up their sleeves.

X-rays: X-rays that look at bones can calculate something called a bone age. It can give a doctor a lot of insight into how a child is growing if the bone age lines up with the chronological age (i.e., your bones look their real age). X-rays also look at growth plates at the ends of the bones. While an adult who is done growing has “closed” or “fused” plates, a child with growth potential will have lots of space in those growth plates.
Tanner staging: When doing a physical exam around those puberty/adolescent years, a doctor can tell a fair amount about development and further growth potential depending on how developed a child is in puberty (specifically looking at genitals and chest/breast development).

As you go in for regular well-visits with your pediatrician, he or she should review with you how your child is growing. Don’t be bashful about bringing up any concerns you may have about your child’s growth.

One of the most important parts of the well visit is the growth curves. A 10-second review of growth curves tells the pediatrician volumes of information. Hopefully, by the time you finish reading this article, you’ll feel like an expert at interpreting growth curves as well.

The growth curves are the little graphs that tell you where your child stacks up size-wise compared to other kids the same age. Now, there are different graphs for boys vs. girls, and different graphs for different countries. It makes sense when you think about it; average kids in China are generally smaller than the average kids in the United States.

What are the different curves?

Younger than 2 years-old:

Height: Measured from the top of the head (with the chin in neutral position) to the bottom of the heel. In our office, we lay a young child down on a paper-lined table and make the marks with a pen. We then move the child and measure the distance between the two marks.
Weight: A more sensitive infant scale is typically used in the younger age groups.
Head Circumference: Measurement around the head (going across the forehead and temples and kept perfectly level).

Older than 2 years-old:

Height: Usually measured with a child standing, without shoes, heels against the measuring wall, and chin level.
Weight: Typically taken on a standing scale.
Body Mass Index (BMI): A mathematical calculation of the body mass divided by the square of the body height. It essentially tells you if your child is overweight, underweight, or just right.

What do the numbers mean?

The numbers tell you where your child’s size is in relationship to other children. Average is 50%. So anything above 50% reflects being bigger than average and anything below 50% reflects being smaller than average.

It is super important to consider the numbers in relationship to each other. If the numbers (height, weight, and head circumference—if under 2 years-old) are all similar in value, then your child is relatively proportionate. If the numbers are vastly different, then it may reflect a bigger problem.

How do you know if there is a problem?

Children tend to grow a steady rate. If any single curve (height, weight, or head) changes percentiles dramatically (either up or down), then something is wrong. For example: If a child has been growing around the 30% for height, weight, and head, but then comes in with a head measurement of 95%, there is likely something happening in that head or brain (e.g., a mass/tumor, or too much fluid). As another example, consider a child who has been growing at the 70% for measurements. If the next visit, the child’s weight is down to 20%, something is likely wrong (e.g., a metabolic problem, family stress, food insecurity, etc.).
Differences between curves. Generally, children grow proportionately. If one of the curve measures is off, that can indicate a problem. For example, if the height and weight are 25%, but the head is 85%, then the child has a disproportionately large head which may warrant further workup to determine if there is a problem (more than just a genetically big head). As another example, consider the child who is 97% for height and 15% for weight. That is a very thin child. One must consider whether a metabolic problem exists? Is the child getting enough to eat? If on the other hand, the child is 55% for weight and 3% for height, the child also has problem. At first blush, it may be easy to think that the child is ok (average weight), but the child isn’t tall enough to support that weight. In that case, the child is overweight. The pediatrician will consider if the child is eating too much, not active enough, or has some underlying metabolic issue.
Not genetically appropriate. When looking at growth curves, one must always consider the child’s genetic potential. If the parents are 4 ½ feet tall and weigh 80 lbs., I expect a tiny child. A huge child coming from those parents may indicate an excess of growth hormone (which can come from tumors). When I was in residency, there was a child who had a growth hormone deficiency despite being in the 75% for height and weight. The child’s dad was over 7 feet tall and mom was 6 ½ feet tall. The deficiency was detected based on the growth curves. Even though the child was bigger than average, the child was not nearly as tall as one would expect for his genetic potential.

Now the next time you are at a play-group and some parent is boasting about their child who is in the 75% you’ll know what they are talking about.

Tag: growth percentile

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