When I first qualified, we were saying periods were arriving early and we’ve continued to say that over several generations. But now Harvard University in the US is telling us, in more detail, how it’s changing puberty.

Researchers point out that the average age of a girl’s first menstrual period has been decreasing among younger generations in the US, especially those belonging to racial minorities and lower socioeconomic groups. They also found the average time it takes for the menstrual cycle to become regular is increasing.

The Apple Women’s Health Study looks at menstrual cycles, gynaecological conditions, and overall women’s health, and is conducted by Harvard and collaborators. Trends towards earlier periods stretch over the past five decades, but not for different racial groups and socioeconomic groups.

Nor was there good research on trends regarding the time it took for cycle regularity. Participants who enrolled in the Apple Women’s Health Study between November 2018 and March 2023, which was 71,341 in total, ­ reported the age at which they first began menstruating as well as their race and socioeconomic status.

The researchers divided the participants into age brackets. Ages of having a period were defined as “early” (younger than 11 years old), “very early” (younger than nine), and “late” (age 16 and above).

A subset of participants (61,932) reported the time it took for their menstrual cycle to become regular and were divided into five categories: up to two years, between three and four years, longer than five years, hasn’t become regular, or became regular with use of hormones. Another subset (9,865) provided their body mass index (BMI) at their age of starting periods.

Among participants born from 1950-1969, the average age was 12-and-a-half-years, and the rates of early and very early periods were 8.6% and 0.6%, respectively. Among participants born from 2000-2005, the average age of menstruation was 11.9 and rates of early and very early periods were 15.5% and 1.4%, respectively.

Across the two groups, the percentage who reached menstrual cycle regularity within two years of starting decreased from 76% to 56%. Researchers noticed these trends were most pronounced among participants who identified as Black, Hispanic, Asian or mixed race, and who rated themselves as belonging to a low socioeconomic status. The findings also showed that childhood obesity, a risk factor for early puberty and a growing epidemic in the US, could be a contributing factor to earlier menstruation.

“Continuing to investigate early periods and its drivers is critical,” said corresponding author Zifan Wang, a postdoctoral research fellow at Harvard. “Early menstruation is associated with a higher risk of adverse health outcomes such as cardiovascular disease and cancer.”