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Month 6 Worry: How Tall Will My Child Be?

Parenthood really begins as we have this little being handed over to us after delivery. Who does the baby look like? Is that Papa Bob’s nose? Aunt Audrey’s chin? It is all a great mystery, and unless we have a forensic artist in the family to draw us the future, it’s natural to wonder what this little body will become.

What’s the Issue?

One of the most common questions asked in my practice is, “How tall will my baby grow up to be?” This is a question that can be answered with some certainty by looking at genetic potential—that is, how tall the biologic parents are. Your pediatrician can calculate this (called a mid-parental height), and you can figure it out at home, too.

Consider the Numbers

The mean height in America is about 5′ 9½” for men and 5′ 4½” for women. Our genetic programming aims us at a point on the bell curve, and other factors create error or variability to either side of that prediction. Dad is 6′ tall? Well he’s close to the 90th percentile for adult men, meaning he’s taller than about 90 percent of the population. A mere 2½ inches on either side of average encompasses 80 percent of the American adult male population. It is interesting how homogeneous we all are, when you think about it. Click here to visit the Centers for Disease Control (CDC) growth curve charts.

What Parents Can Do

To calculate your son’s genetic potential, take Mom’s height plus 5 inches and add Dad’s height. Now average the two. For example, if Dad is 5′ 10″ and Mom is 5′ 3″ you would take 5′ 10″ and 5′ 3″ plus 5″ (or 5′ 8″) and divide that answer by two. The average of 5′ 10″ and 5′ 8″ is 5′ 9″. That is your son’s genetic potential.
So:

(Mom’s height + 5 inches) + Dad’s height = Son’s genetic growth potential divided by 2

To calculate your daughter’s genetic potential, take Dad’s height minus 5 inches and add Mom’s height. Now average the two. For example, if Dad is 6′ and Mom is 5′ 2″ you would take 6′ minus 5″ (or 5′ 7″) plus 5′ 2″ and divide that answer in half. The average of 5′ 7″ and 5′ 2″ is 5′ 4½ inches. This would be your daughter’s genetic potential.
So:

(Dad’s height – 5 inches) + Mom’s height = Daughter’s genetic growth potential divided by 2

If the math is tripping you up in feet and inches, these calculations may be easier if you convert all the heights to inches first.

What the Docs May Do

Doctors find these calculations helpful when a child is growing more quickly or slowly than expected. It is also helpful when judging the onset of puberty in an older child. As you may know, puberty is occurring at an earlier age these days. The onset of pubic hair or breast budding in girls can occur normally as early as age six! These calculations can help doctors determine whether your child’s early development is in the normal range.

Of course, it is easy to spot exceptions to the rule: While Great Uncle John’s tall genes may have passed unexpressed through the generations, your child may get lucky and express them, towering over the rest of the family. And if your peanut baby is growing at the bottom of the length curve, rest assured. A baby’s early growth pattern, especially with babies born preterm, is a poor predictor of final adult height.

When doctors need a more exact height prediction, they obtain a hand and wrist X-ray (or “bone age”) when a child gets a little older. Doctors compare the growth plates in the left hand and wrist to a book of standards. In this way, they can tell whether your child is an early or late bloomer and by how much. This testing is usually ordered by your doctor only if there is a medical indication.

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