Still, she added, many girls who develop early are not overweight.
“Obesity can’t explain all of this,” Dr. Shaw said. “It’s just happened too quickly.”
Chemicals
In the decade after the Herman-Giddens study, Dr. Juul began noticing an increase in the number of referrals for early puberty in Copenhagen, mostly of girls who were developing breasts at 7 or 8 years old.
“And then we thought, ‘Is this a real phenomenon?’” Dr. Juul said. Or, he wondered, had parents and doctors become “hysterical” because of the news coverage of Dr. Herman-Giddens’s study?
In a 2009 study of nearly 1,000 school-aged girls in Copenhagen, his team found that the average age of breast development had dropped by a year since his earlier study, to a little under 10, with most girls ranging from 7 to 12 years old. Girls were also getting their periods earlier, around age 13, about four months earlier than what he had reported before.
“That’s a very marked change in a very short period of time,” Dr. Juul said.
But, unlike doctors in the United States, he did not think obesity was to blame: The body mass index of the Danish children in the 2009 cohort was no different than it had been in the 1990s.
Dr. Juul has become one of the most vocal proponents of an alternate theory: that chemical exposures are to blame. The girls with the earliest breast development in his 2009 study, he said, had the highest urine levels of phthalates, substances used to make plastics more durable that are found in everything from vinyl flooring to food packaging.
Phthalates belong to a broader class of chemicals called “endocrine disrupters,” which can affect the behavior of hormones and have become ubiquitous in the environment over the past several decades. But the evidence that they are driving earlier puberty is murky.
