This week, I've been reading and re-reading the latest JAMA paper, which found a decrease in the average age of menarche (a girl's first period) from age 12.5 in the 1950's and 60's down to age 11.9 for girls born between 2000-2005.

Well, that's the headline that has been going around in my "corner" of the internet, but the study is interesting for a number of different reasons, so I wanted to take time to share a summary and some quick thoughts with you here!

WHAT WAS THIS STUDY?

The Apple Women's Health Study is a collaborative effort between Apple, the Harvard T.H. Chan School of Public Health and the National Institute of Environmental Health Sciences, that allows users of the Apple Research App to participate in data collection for various women's heath initiatives. This includes efforts to better understand PCOS, menopause, and-- in this case-- menarche. Users of the Research app were asked a series of questions about their menarche experience along with demographic information. Some users were even able to provide specific medical information which allowed the research team to calculate their approximate BMI at the time of menarche.

This particular study specifically asked women (ages 18+) to recall:

1. What age they first started menstruating
2. How long it took their cycles to become regular

The respondents were grouped into FIVE age categories, based on their birth year:

• 1950-1969
• 1970-1979
• 1980-1989
• 1990-1999
• 2000-2005

WHAT DID THEY FIND?

The major headline, as I said, was the finding that average age of menarche has been decreasing over the past 50+ years. In the first age group, the average age a girl got her first period was found to be 12.5. This is close to the often-cited statistic of mean age being 12.4 in the US (as seen recently in this 2023 article). But according to this new data, we haven't been at an average age of 12.4 for a while. Starting with the 1980's cohort, the average age ticked earlier and earlier until we see an average age of 11.9 years at menarche with the girls born in the early 2000's.

Because this data was limited to women aged 18+, it's entirely possible that girls born the following 5 years might be even earlier. We just wouldn't know that yet.

So yes: girls in the US are getting their periods at a younger average age... but that doesn't mean everyone is getting their periods earlier!

Average ages are calculated based on.... averages. So the interesting thing to note with this data was not the overall number. I think the major story here is that the percentage of girls entering early and very early puberty is skyrocketing, which of course is impacting the average.

Specifically, the number of girls entering early menarche (defined as a first period before age 11) increased from 8.6% of girls in the 1950's and 60's to 15.5% of girls born in the early 2000's. That's almost double!

But the number of girls entering VERY early menarche (defined as a first period before age 9) MORE than doubled, increasing from just 0.6% in the first age group to 1.4%.

Another interesting finding was that only a small percentage of women born in the 1950's and 60's felt their cycles never really got "regular." They made up about 3.4% of the respondents in that category. The vast majority (over 75%) of women said their cycles were pretty regular within a couple of years, and about 10% of them said it took between 3-5 years to see regularity.

This was not anywhere near the breakdown with women born 1990 and after.

For those women, only about half of them said their cycles were regular within the first couple of years. Around 10% still said it was probably more like 3-5 years before their cycles got regular, but over 14% of women said their natural cycles were STILL not regular. This is almost a 5x increase!!!!!

LET'S HIT THE BREAKS FOR A MINUTE

This study does have some interesting limitations, some of which are discussed by the researchers and some which seem to be completely overlooked. So before we all go sounding some crazy alarm warning bell, let's take a breather and acknowledge:

1. While most girls and women do have good recall of our age when periods started, I'm not so sure that applies to recollections of when our cycles became "regular." This sort of recall bias is mentioned in the study, but lightly dismissed. What is NOT discussed is that recall accuracy might vary a lot based on age, not just because our memories fade but because certain generations might have had very different attitudes about periods, and therefore kept mental note of this aspect of puberty in very different ways.
2. Something completely omitted in the conversation was a definition of "regularity." If they provided respondents with a medical explanation of cycle regularity, I cannot find that in this paper. Which seriously begs the question: "Could these statistics be skewed by shifting views of what a regular menstrual cycle is?"

I cannot tell you how many young women ask me about helping them chart their "very irregular" cycles; when I ask to see some of the data, they show me cycles ranging from 29 to 33 days... WHICH ARE PERFECTLY NORMAL.

Part of me seriously questions the value of this aspect of the data, because I am willing to bet that women who were born in the 1950's and 60's were not yet primed to believe that a "regular" cycle meant you had to have a period every 28 days. Women born after the popularization of the pill likely have VERY different expectations for their periods, which could be much more rigid than previous generations. This could certainly be one aspect in the large difference between these numbers.

But we do also know that reproductive issues like PCOS (which often occur with irregular cycles) are becoming more prominent in the general population. So we can't dismiss that statistic outright. I just mention it because it's an obvious situation in which a woman's perception of "regularity" and therefore her assessment of her own cycles may not be well-informed. Which means... we need better education.

WHAT DOES THIS MEAN?

This study simply looked at trends, and can't tell us about any particular mechanisms. What the data did suggest is that higher BMI may be the culprit for about half of these instances of early or very early menarche, but obviously lots more research is needed. Other contributing factors could be endocrine disruptors, pollutants, diet, stress, and trauma.... just to name a few!

So from where I sit, the obvious meaning to draw from this study is the NEED to prepare parents, educators, girls, and peers for the start of puberty and menstrual cycles SOONER and in more age-sensitive ways than most of our models currently assume.

This is going to sound like an advertisement. I know. There are a lot of things we might learn in the following years about specific things we can do to decrease the chance of early menarche for our girls. We just don't know what those things are right now, and how much of a difference it might make.

But what we CAN do is be proactive about educating our girls, so they have a really solid understanding of what cycles and periods are, how they contribute to many aspects of our growing and changing bodies, and what is actually healthy and regular to see with cycle patterns. If we do this, they are not only better prepared for themselves, but they are better prepared to be really good, caring, attentive friends to any of the girls in their classes who may be going through this at very young ages.

This is how we do better.

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