Girls in the United States have been getting their periods at younger ages over the last fifty years, with additional variation between races, according to a new study.
The study published in the JAMA Network on Wednesday found that the average age that girls got their periods for girls born between 2000 and 2005 was 11.9 years, which is younger than the average age of 12.5 years for those born between 1950 and 1969.
It also found that participants who self-identified as Asian and non-Hispanic Black were more likely to report earlier menstruation. Menarche, which refers to the first-time girls get their period, typically occurs between the ages of 11 to 15, according to the Cleveland Clinic.
Girls are also reporting longer lengths of time until their period cycles become regular. The number of individuals who reported regularity with their cycles within two years of starting their periods dropped over the past five decades.
Zifan Wang, the study’s lead author, told The Washington Post that it was “concerning” that the study showed girls taking longer to have regular menstrual cycles.
“This is also very concerning because irregular cycles are an important indicator of later-in-life adverse health events. It alarms us. We need to do more early counseling and intervention on irregular cycles among children and adolescents,” Wang said, according to The Post.
The study of 71,341 individuals was conducted as a part of the Apple Women’s Health Study, which used cycle tracking data from iPhones and Apple Watches as well as surveys to learn more about menstrual cycles.
The researchers noted that there were some limitations to their study and that their results may not be generalizable to all U.S. individuals who menstruate. They said potential selection bias may arise due to participants opting into the study.
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