US girls got their first periods increasingly earlier over the last 50 years, new study finds

<span>The study found that women born in the oldest bracket, between 1950-1969, got their period at 12.5 years old on average, compared to 11.9 years old for the youngest group, born between 2000-2005.</span><span>Photograph: SDI Productions/Getty Images</span>
The study found that women born in the oldest bracket, between 1950-1969, got their period at 12.5 years old on average, compared to 11.9 years old for the youngest group, born between 2000-2005.Photograph: SDI Productions/Getty Images

Girls in the United States had their first periods earlier over the last five decades and it took longer to experience regular cycles, a new study has found.

The study, published in JAMA Network Open, found the trend is especially pronounced among Black, Hispanic, Asian and mixed race participants, and among those who reported lower socioeconomic status.

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

Beginning in 2019, researchers from Harvard and the National Institute of Environmental Health Sciences (NIEHS) surveyed more than 71,000 participants born between 1950 and 2005 about when they got their first period, when it became regular and for certain demographic information. Researchers then divided the group into five generational brackets.

They found that women born in the oldest bracket, between 1950-1969, got their period at 12.5 years old on average, compared to 11.9 years old for the youngest group, born between 2000-2005.

The study was conducted through an app as part of the Apple Women’s Health Study. This has allowed researchers to check back in with a group multiple times, or what researchers call a longitudinal study design.

Although the study was large, it relies on self-reported information – which is generally considered less reliable than sources such as medical or financial records. In some cases, it would have required participants to think back decades. Still, the study will likely provide direction for future research.

In addition to a younger average age of menarche, the study found the rate of people getting a first period early (younger than 11) or very early (younger than nine) roughly doubled between the oldest and youngest generations. For the oldest group, 8.6% got a period before 11 years old, compared to 15.5% of people in the youngest generation. Similarly, 0.6% of people in the oldest generation got a period before age nine, compared to 1.4% in the youngest generation.

“We certainly see patients who are presenting with periods at age nine or 10,” said Dr Anne-Marie Amies Oelschlager, a pediatric gynecologist at the University of Washington School of Medicine, and the chair of the American College of Obstetricians and Gynecologists (ACOG) clinical consensus committee for gynecology.

She continued: “Even though that’s considered in the typical range, it’s quite distressing for our patients and their parents. They’re still in elementary school in third or fourth grade, and trying to manage periods in third or fourth grade is hard.”

Importantly, menarche at younger ages may not be pathological. Amies Oelschlager said better sanitation and nutrition are likely at least part of the reason girls are getting earlier periods, both in the US and globally.

“If someone is showing signs of puberty before age eight they should talk to their doctor about it, and if they haven’t had a period by age 15 they should also talk to their doctor about it,” she said. In some cases, very early puberty can be a sign of rare but serious conditions, such as brain tumors.

An early period can have lifelong implications. The onset of puberty fuses growth plates, meaning people who hit puberty early may not reach their maximum genetic height. It can also elevate women’s risk of cardiovascular diseases and breast cancer.

Early physiological development can also have dramatic social impacts, because it does not coincide with early cognitive development. Children who experience early puberty are at increased risks for sexual violence, sexually transmitted infections and early pregnancy.

Early puberty is also associated with increased risk of depression, anxiety, substance abuse and suicidal behaviors.

Amies Oelschlager said that the literature presently suggests breast development is occurring in younger American girls, but further research will be needed to confirm that menarche is occurring on average in girls younger than 12, as the new JAMA Network Open study suggests.

Many factors influence the onset of puberty, but the exact reasons for menarche and breast development at younger ages is debated. One hypothesis is that higher body fat percentage triggers the pituitary gland to produce puberty hormones. Other research has shown that body mass index is the greatest predictor of early menses. Scientists reason that a greater prevalence of childhood obesity may explain the higher proportion of girls getting early periods.

“What we need to ask is, why has [body mass index] gone up?” Dr Frank Biro, a physician at Cincinnati Children’s Hospital Medical Center, told Scientific American. “Decreased physical activity and a more calorically dense diet are probably part of the puzzle. But I think another critical piece is our ubiquitous environmental exposure to endocrine-disrupting chemicals.”

Endocrine-disrupting chemicals from plastics and petrochemical pollution are found in a wide array of consumer goods – including pesticides, construction materials, furniture, children’s toys, fabrics and cosmetics. Most remain unregulated despite scientists’ warnings about potential negative effects on human health.

Wang said a wide array of factors need to be considered to understand why menarche may be happening to younger girls.

“These factors may include what’s in the environment like chemicals that affect hormones and air pollution, or dietary patterns, stress, and adverse childhood experiences,” she said. “Studying these factors could help us find better ways to stop or slow down these trends.”