Today’s young people are hitting puberty earlier and starting menstruation younger. New research reveals that girls in the U.S. started their periods earlier over the last five decades and experienced delays in regulating their cycle.

The number of girls who got their period before age 9 doubled, according to the study published in JAMA Network Open on May 29, with Black, Hispanic, Asian, mixed race and low income youth are more likely to experience early menstruation.

For participants born from 1950 to 1969, the average age of menarche, or their first period, was 12.5 years, with only 9.2% experiencing early or very early menarche, before age 9. Comparatively, participants born between 2000 and 2005 on average hit menarche at 11.9 years, with 16.9% experiencing early or very menarche.

“This is important because early menarche and irregular periods can signal physical and psychosocial problems later in life,” said Zifan Wang, a postdoctoral research fellow at Harvard University TH Chan School of Public Health and lead author of the study.

Periods are major indicators of a person’s health, and the age of puberty is used to measure population health by researchers and entities like the World Health Organization. Generally, countries with a mean menarche age older than 12 face nutrition issues and higher infectious disease rates, while countries whose mean menarche age is younger than 12 face overnutrition and higher risk of chronic diseases, says Lauren Houghton, assistant professor of epidemiology at Columbia University.

“The US is an example of the latter, and the decreasing age of menarche points to possible increases in chronic diseases, including mental health, cardiovascular disease, and breast cancer,” Houghton wrote in commentary accompanying the study. “Just as clinicians regularly check blood pressure, they should also inquire about menstrual health, including last menstrual period, cycle length and regularity, and associated symptoms that occur around menstruation. Menstrual cycle characteristics outside clinical norms or changes over time for an individual may be a warning sign of underlying risk and pathology.”

Obesity, stress and cancer?

Evidence shows early menstruation points to lifelong health issues, and the health concerns researchers believe may be driving younger onset puberty, may also indicate problems in adulthood. One indicator researchers suggest could cause children to get their periods younger and lead to other health issues later in life is high BMI.

“Childhood obesity might be a key factor. Our exploratory analysis suggests BMI at menarche accounts for 46% of the trend toward earlier menarche,” Wang posted on Twitter/X in a thread about the study.

Childhood obesity is a known risk factor of early puberty, according to the National Institutes of Health (NIU), with the rate of childhood obesity tripling since the 1970s. In April, the Centers for Disease Control and Prevention reported that 1 in 5 U.S. children and adolescents were obese, with higher prevalence in Black and Latino youth.

While weight loss can reduce the risks associated with childhood obesity, Most obese children remain obese in adulthood, according to Georgetown University’s Health Policy Institute, increasing their chances of being diagnosed with four of the leading causes of the death in the U.S., heart disease, type II diabetes, stroke and cancer.

In her commentary, Houghton suggested the pandemic stay-at-home mandates required less physical activity in children and was linked to stress, which could be another driver causing earlier puberty.

“It is very likely that BMI and stress interact to accelerate age at menarche and prolong cycle regularity. My working hypothesis is that more stress increases the pool of [sex hormones] androgens and more adipose tissue [body fat] converts the abundance of androgens into estrogens, leading to earlier puberty,” wrote Houghton. “Despite the biology, there is little research that simultaneously investigates mental and physical drivers of menarche and menstrual cycle regularity.”

In March, Stat reported that girls who experience early puberty are more likely to be bullied and have higher rates of depression and other mental health affects that are carried into adulthood. Aviva Sopher, a professor of pediatrics at Columbia University told Stat News that getting your first period at a young age could cause future stress itself.

“With more girls going through puberty earlier, it might make it more comfortable for those experiencing it,” Sopher said. “But on the other hand, in general, girls who go through puberty earlier are at increased risk of sexual abuse because they look very mature, but they’re immature.”

Wang said that the key to unlocking answers as to what’s causing early menstruation, and what this means for women and girls in the future is funding.

“To address these health concerns—which our findings suggest may begin to impact more people, with disproportionate impact on already disadvantaged populations—we need much more investment in menstrual health research,” said Wang.