What's the fastest a human can grow?

If you have teenagers — or were once a teenager yourself — you've probably experienced this: Your kid's jeans fit perfectly in September, but by December they're showing ankle. The adolescent growth spurt can feel astonishingly fast, with some teens growing 4 to 5 inches (10 to 13 centimeters) in a single year. But is this really the fastest period of human growth?

Surprisingly, no: The teenage growth spurt is only the second fastest that humans can grow.

"We've been tracking children's growth from birth all the way through to adulthood, and the fastest period of growth is clearly the first couple of years during infancy," Sean Cumming, a professor in the Department for Health at the University of Bath in the U.K., told Live Science. "That's when children are growing most rapidly."

Babies can add nearly 1 foot (25 to 30 centimeters) of height per year — more than double the rate of even the most dramatic teenage growth spurts.

In fact, for girls, "at 18 months, they will be 50% of their adult size," Adam Baxter-Jones, a professor in the College of Kinesiology at the University of Saskatchewan in Canada, told Live Science. Boys reach 50% of their adult size at 24 months, he said.

Then, things slow down. "When we get into late infancy and childhood, physical growth is put on the back burner," Cumming said.

Growth drops to about 2 to 2.5 inches (5 to 6 centimeters) per year from age 4 until puberty, according to Baxter-Jones. That's when humans hit their second-fastest period of growth.

At their peak during puberty, girls grow an average 3.5 inches (9 centimeters) per year, and boys grow an average of 4 inches (10 centimeters) per year, according to a study in the Journal of Adolescent Health.

But those average peak growth rates are just that — averages.

"If we measure regularly enough, what we see is these periods of really, really intense growth, and then the body adapts afterward," Cumming said. "We can see rates of anywhere up to about 20 centimeters [nearly 8 inches per year] in some of the studies that we've looked at … of course, if you average that over a period of time, [you get] 10 to 12 centimeters [4 to almost 5 inches] per year."

Just as they did when they were babies, girls go through their growth spurts earlier — around 11 years old, while boys usually enter puberty around two years later.

"Boys will typically have a slightly more intense growth spurt," Cumming said. "That's because they produce more growth hormone, but also testosterone, which is also contributing to bone length."

The pubertal growth spurt stops at around age 16 for girls and 18 for boys — and because boys both have that more intense growth and grow for around two years longer, they end up taller on average.

The age at which someone hits their growth spurt doesn't affect their final height — someone who matures early stops growing sooner than someone who matures later, so someone who matures late has more time to grow, Baxter-Jones said.

Growth spurts and body shape

Growth spurts happen from the outside in. "First it's the feet and the hands, and then it's the long legs and the long arms. That's why you see kids just at the start of puberty, they look like baby giraffes. They've got these big clown feet, these legs that go on forever," Cumming said.

The torso grows last — and if a child develops late, sometimes the torso never quite catches up with the rest of the body. As a result, in sports like ballet and gymnastics, teams select for late developers because they have a more linear physique and longer legs, according to Cumming. However, early developers have their own athletic advantages.

"If you get a puberty growth spurt early, you're bigger, stronger. Those are the kids that get selected for all the top positions and into the top academies," Cumming said. "In the Scottish Academies we surveyed over a thousand kids beyond 14 years of age. We didn't find any late developers at all."

But that rapid growth comes at a cost. During growth spurts, bones are weaker and more susceptible to damage.

"Your bones grow, and then they mineralize. There's about a nine month gap there," Baxter-Jones said. "The peak fracture rate is during that adolescent growth spurt."

Muscles and tendons also take up to nine months to catch up with the growing bones, which can lead to growth-related injuries, especially around the heel, knee and lower back.

However, monitoring growth spurts carefully can help prevent more serious problems. "If we do that in the Premier League academies, we can reduce those non-contact injuries by about 70%," Cumming said.

For parents wondering if their child's growth pattern is normal, both experts emphasized that wide variation is expected.

"It's normal to grow quickly, but it's also normal to grow slowly," Baxter-Jones said. Final adult height comes down to genetics. There are also rare conditions in children, such as pituitary gigantism, that lead to excessive production of growth hormone. Children with this condition can grow up to 6 inches (15 cm) a year, and one report documented a 13-year-old boy growing 7.5 inches (19 cm) a year. But even this fast growth is less than the rate at which babies grow.

So what's the fastest a human can grow? The answer isn't when you're stretching out of your jeans as a teenager — it's when you were too young to remember it happening at all.