Linlin Liu: What Counts as “Normal” for Your Menstrual Cycle?

Linlin Liu, award-winning designer in consumer-facing technology and products, shared on LinkedIn:

‘I paused at the data selection section in the Stanford University and WHOOP study of 42,759 menstrual cycles, and found myself going down a rabbit hole about how the ‘normal’ reference population gets defined.

‘The OB-GYN told me that was normal.’

I’ve heard some version of this again and again when talking with women about their periods.

My mind keeps circling one question: what does ‘normal’ actually mean when it comes to the menstrual cycle?

I spent almost two weeks on this and wrote it down to work through it.

Sharing what I learned and the questions I still have. If you work in menstrual or reproductive health, or building in this area, I’d genuinely love your perspective.

What counts as ‘normal’ for your menstrual cycle?

‘I just thought that’s normal.’

‘The OB-GYN told me that was normal.’

Almost every woman I’ve talked to about their period-related issues said this when I asked why they didn’t look for help or solutions when they experienced discomfort and pain.

In some of the cases it might be true. Women ended up enduring the pain without severe consequences. In others, the discomfort kept going and later developed into serious cases that required major surgeries, proving what they experienced was absolutely not normal.

So what defines ‘normal’? And when we talk about menstrual cycles, what is ‘normal’ being compared to?

A study of 42,759 menstrual cycles from 2,596 women, with 1.3 million days of wearable data from researchers at Stanford and WHOOP, caught my eye. Aside from the catchy memory point that women sleeping about 6 hours had 1.3x the odds of a cycle shifting by 3+ days compared with those who slept 8, it was the methods section (how the data was selected) that made me stop and think.

Decide a ‘normal’ baseline

The researchers started with 12,010 participants. They ended with 2,596. From the paper, the people who were filtered as follows:

• ‘excluded if they reported using hormonal birth control’
• ‘Participants who logged pregnancy or menopause symptoms… were excluded’
• ‘we only included participants whose median cycle length was between 21 and 35 days’
•  plus: ‘at least 75% nightly device wear, at least 90% of expected cycles logged, a minimum of five cycles’ (an indicator that they are actively managing their health)

In the end, 78% of the starting cohort was removed.

It fascinated me how they set up a baseline for the study. Yes, to study menstrual cycles scientifically, you first have to define a baseline and decide who belongs in the reference population, so that you can draw conclusions referring to that population.

It also means your study quietly draws a line. Above it is ‘regular.’ Below it are women with irregular cycles, diverse health conditions, varied socioeconomic backgrounds (obviously, these are not able to afford the device and the subscription), or different ancestries.

I can’t stop thinking: when companies build solutions based on that ‘normal’ reference population, what do they offer to the 78% who didn’t make it in?

Compared to what?

When an OB says your cycle is normal, I think there are two ways to compare:

• Compared to other women. Just to count as ‘regular’ in this study, your median cycle length had to fall between 21 and 35 days. That’s a fourteen-day-wide, two-week-long range. A woman with a 22-day cycle has about six more periods every year than a woman with a 34-day cycle. And that’s only counting how often she bleeds, before you add the variance in how many days she bleeds each time.

Is it just me, or is that a lot of variance to still be called normal?

• Compared to your own history. In this study, ‘27% of cycles deviated by three days or more from a participant’s median’. These are women selected for having regular cycles. More than a quarter of their cycles still didn’t match their own typical length. Even her own ‘normal’ baseline isn’t a steady line.

And a woman’s cycle changes with the stage of her life – as a teenager, in her twenties, during a stressful year, after a cross-country move, after a baby, etc. Nobody is keeping that record or connecting all the dots to compare her to.

And for everyone filtered out, neither comparison exists at all.

A woman on the pill doesn’t have a menstrual cycle in the sense being measured. What she has monthly is what clinicians call a withdrawal bleed, which doesn’t mean she ovulated, which means it is not a real period in the physiological sense. A woman with a hormonal IUD may bleed much less, or not at all.

They can’t be told they’re normal. They can’t be told they’re abnormal either.

To be fair, the authors name most of these limits in their own limitations section. So I hope whoever builds solutions based on this research will consider the women outside that ‘normal’ reference population too.

Claude Lévi-Strauss, the anthropologist, said, ‘The scientist is not a person who gives the right answers; he’s one who asks the right questions.’

I’m not a scientist, but after going down the rabbit hole of what counts as a ‘normal’ menstrual cycle, I do want to throw these questions out:

What should ‘normal’ mean, both personally and in general? How much variance is fair to call normal? And what would a baseline built for her, not for the average, have to look like?

If you are a medical expert in menstrual health, reproductive health, or build in menstrual health, I’d genuinely like to hear your answers and perspectives!!”

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