Researchers looked through school health records and assessed trends in height and growth for 4,090 boys born in Sweden from 1947 to 1996. Boys born later in that 50-year time period hit puberty sooner. For every decade born later, boys reached peak height velocity, or PHV — the study’s marker for puberty — 1.5 months earlier.
The age at PHV became progressively younger for boys born later, dropping from about 14.2 years in 1947 to 13.7 years in 1996.
“It’s a big difference … that’s middle school versus high school,” said Dr. Cora Breuner, professor of adolescent medicine and orthopedics and sports medicine at Seattle Children’s Hospital, who’s also on the American Academy of Pediatrics Committee on Adolescence. Breuner was not involved in the study.
Part of the earlier puberty trend could be attributed to increases in childhood BMI over time, the study found. Increases in childhood weight, based on body mass index calculations at age 8, were related to earlier pubertal timing in boys, but didn’t explain the full picture. (Body mass index is a person’s weight in kilograms divided by the square of height in meters. For children and teens, BMI is age- and sex-specific.)
Even after adjusting for childhood body mass index, the researchers saw that the age of PHV was 1.2 months earlier for every decade born later.
Other factors for earlier pubertal timing could be nutrition, socioeconomic environment or exposure to chemicals that disrupt hormones, the researchers cited, but they didn’t directly assess those.
Among the limitations to the study: Participants were a relatively homogenous group of white, Swedish boys with a body mass index ranging from 12.3 to 29.3 — lower than the range for US boys.
These findings can only be “cautiously extrapolated to a heavier and more heterogeneous population of US adolescent boys,” Dr. Vanessa Curtis from the University of Iowa and Dr. David Allen from the University of Wisconsin wrote in an editorial that published alongside the study.
Study has limitations
This isn’t the first study to find earlier onset of puberty in boys. A 2012 study showed boys in the United States sexually develop six months to two years earlier than what medical textbooks say is standard. But in that study, genital size and sperm production were used as measurements for puberty, not PHV. While puberty in girls is usually marked by the age that they first start their period, an objective measurement for puberty onset in boys hasn’t been as established across studies. PHV indicates the age of maximum growth spurt, which usually happens two years after puberty starts in boys. The study’s researchers evaluated PHV as a marker for puberty in a previous study.
“For boys, there is no reliable corresponding marker for puberty and therefore studies on trends in pubertal age among boys are scarce,” researchers Maria Bygdell, Dr. Claes Ohlsson and Dr. Jenny Kindblom from the University of Gothenburg in Sweden said in an email.
Furthermore, the PHV may not be an accurate measurement, as researchers couldn’t measure secondary sex characteristics, such as voice changes or hair growth, to confirm onset of puberty. The PHV also couldn’t measure if boys hit sexual maturity earlier.
The researchers didn’t look at trends beyond 1996, but since that time, teenagers have had more exposure to screen time through devices and have had more mental health issues, which need to be considered in future studies, according to Breuner.
However, these findings could still guide pediatricians, who may need to be “more ready to discuss” things like normal body changes, drug and alcohol use, sex and mental health “at a younger age,” Breuner said.
And while the study showed earlier pubertal timing for boys, it’s still not as early as that of girls, who usually hit puberty two years sooner, a fact that’s been established in previous studies, according to Breuner. Earlier puberty in girls has been linked to increased risk for diabetes and breast cancer.
The long-term effects for earlier puberty in boys are still unknown and will be important to address, the researchers noted.