Girls are starting their menstrual cycles at an earlier age than ever before. The shift is even more striking among minority groups, adolescent women from lower-income backgrounds and those with a higher body mass index (BMI), according to a new study out of Harvard T.H. Chan School of Public Health. The finding adds to mounting evidence that girls are reaching puberty at an ever-younger age.

More than 71,000 women born between 1950 and 2005 agreed to share health information through an iPhone app for the study. Their stated ethnicities included Black, Hispanic, Asian and white. The women also reported their socioeconomic status, identifying it as low, medium or high.

Almost 16 percent of women born between 2000 and 2005 were between nine and 11 years at menarche. This is nearly double the incidence among women who were born between 1950 and 1969.

The research showed:

  • Overall, while the median age at menarche hasn't fallen too drastically (landing somewhere near 12 years), the share of women who started their menstrual cycles when they were younger than eleven increased significantly over time.
  • Almost 16 percent of women born between 2000 and 2005 were between nine and 11 years at menarche. This is nearly double the roughly 8 percent incidence among women who were born between 1950 and 1969.
  • More women experienced irregular cycles for 3 years or longer after the start of menarche.
  • Black, Hispanic and Asian women were more likely to experience earlier menarche age than were white women. This was also true for those with lower socioeconomic status.

“Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign,” co-principal investigator, Shruthi Mahalingaiah, assistant professor of environmental, reproductive and women's health at Harvard Chan School, said in a press release.

Being overweight — having a high BMI — was seen as responsible for slightly less than half of the declines in girls' age at first menarche. The researchers suggest that in addition to BMI, the other factors that could contribute to the lower age at the start of menarche might be sugar consumption and other dietary influences, psychological stress and traumatic childhood experiences, and environmental factors such as exposure to endocrine-disrupting chemicals and air pollution.

“Early menarche is associated with higher risk of adverse health outcomes, such as cardiovascular disease and cancer. To address these health outcomes — which our finding suggest may begin to impact more people, with disproportionate impact on already disadvantage populations — we need much more investment in menstrual research,” corresponding author, Zifan Wang, a postdoctoral research fellow in Harvard Chan's Department of Environmental Health said. “Continuing to investigate early menarche and its drivers is critical,”

The study is published in JAMA Network Open.