Menstrual cycle length and variability: a visual explanation

Study Update, September 2023

Part 3

What is an irregular cycle?

In Part 2, we looked at the median of each person's recorded cycles (their "typical" cycle), and how that varies a lot between people in the study.

Another way to look at someone's cycles is to consider how much they vary from one month to the next, or how regular they are. You might have heard someone say they have "irregular" cycles. In this section, we're defining what that means and how common it really is.

You are entering a section with a fixed visualization graphic in the background and short text snippets that appear in the foreground one at a time as you scroll down the page. The graphic updates with each new text snippet.

Let's bring back our familiar stack of cycles.

Graphic: There are five rows of small squares, each row representing a cycle. Each row of small squares begins with 4-6 red squares to indicate the period, and is then followed by grey squares to indicate the non-bleeding days in that cycle. From top to bottom, the cycles have period length and total cycle length, respectively, of 5 days and 30 days, 6 days and 24 days, 5 days and 28 days, 4 days and 27 days, and 6 days and 29 days. If you’ve read parts 1 and 2, this is the same visual representation from the end of part 1 and the beginning of part 2.

Now, rather than taking the median of the cycle lengths, we're going to compare the lengths of consecutive cycles.

Graphic: The visual zooms into the right side of the stack of cycles. The left hand side fades out. Each row is labelled with its total length: 30 days, 24 days, 28 days, 27 days, 29 days.

For example, the second cycle is 6 days shorter than the previous one.

Graphic: The labels showing the cycle length disappear. 6 light orange squares appear to the right of the second row from the top, showing that the second cycle is 6 days shorter than the first one. These orange squares are labelled -6 days.

The third cycle is 4 days longer than the second one.

Graphic: The last four days of the third cycle are highlighted dark orange, vertically below the light orange squares, showing that the third cycle is four days longer than the second one. These squares are labelled +4 days.

We call these cycle length differences, and they measure how regular or irregular a person's cycles are.

Graphic: One light orange square appear to the right of the fourth row of squares to show that it is 1 day shorter than the third cycle. The last 2 squares in the bottom cycle are highlighted dark orange to show that it is 2 days longer than the fourth one. These stacks are labelled -1 days and +2 days.

This measure of cycle variability is not the same as the one shown in Health app. Any measure of cycle variability will depend on which cycles are included in the calculation.

Whether cycles are longer or shorter doesn't matter when talking about cycle length differences.

Graphic: The rows of light and dark orange squares merge into one single orange square for each row, and drop down onto an x-axis. The numbers from the labels (1, 2, 4, 6) also drop down to label the x-axis. The axis title reads Cycle Length Difference (Days).

Similar to the previous part, we're going to take the median of these values. The median cycle length difference gives us one number to measure how regular or irregular a person's cycles are.

Graphic: The orange squares move together to the 3 days point on the x-axis, the median value, and become a circle.

If a person's median cycle length difference is 0, then all of their cycles are exactly the same length. This is the most "regular" a person's cycles could possibly be.

Graphic: The 0 days point on the x-axis is labelled, and a vertical dashed line highlights the axis limit.

There is no perfect definition of what makes a person's cycles "irregular." One way to define it is that your median cycle length difference is 9 days or more. This means that half of the time one cycle is longer or shorter than the next by 9 days or more.

Graphic: The axis zooms out and the 9 days point is labelled. An orange highlighted region extends from 9 days to the right edge of the page.

Let's bring back our 100 imaginary people who are representative of the participants in the AWHS. This time, how many of those people do you think would have irregular cycles?

10 out of 100 people

Graphic: 100 circles in a grid appear at the top of the plot area. 10 of these circles are orange, and the remaining 90 are grey. As you adjust the slider, the number of orange circles will always reflect the slider value.

orange circle is 1 person

You guessed 10. It's actually much more!
23 out of 100 people would have irregular cycles.

Graphic: 23 of the circles drop down from the grid onto the x-axis in the orange region that extends from 9 to the right edge of the page into a quantile dot plot. The dots are stacked in 1 day bins on the x-axis. 2 dots are at 9 days, 3 dots are at 10 days, 1 dot is at 11 days, and 2 dots are at 12 days. The remainder of the dots are spread out along the x axis with at most one dot per bin.

Of those 23 people, 21 would have median cycle length difference between 9 and 40 days.

Graphic: The axis zooms out so that 9 days, 20 days, and 40 days are visible on the x-axis. 21 dots are between 9 and 40 days. One dot is above 40 days, and 1 dot is not visible but is off the page to the right.

The remaining 77 people would have regular cycles, but there is still a lot of variation in median cycle length difference.

Graphic: All 100 circles are shown on the x-axis in a quantile dot plot. The graph peaks at 2 days, where there are 23 dots in a stack. 10 dots are at 1 day, and 1 dot is at 0 days. 13 dots are at 3 days, 12 at 3 days, and 6 dots are at 5 and 6 days, each. 3 dots are at 7 and 8 days, each.

It's very uncommon to have cycles that are always exactly the same length. Most people’s cycles vary in length from one to the next with a median difference between 1 and 6 days.¹

It's important to understand how much variability you should expect in your cycle so that you know when it might be time to talk to your doctor. Cycles that are unusually long (longer than 40 days) or are very irregular can be linked to health conditions that need attention. If you are experiencing discomfort from your periods, that is also a good time to speak with your doctor.

This is the end of the section with the visualization graphic.

In the final part, we will discuss more about why your cycle changes throughout your lifetime, and why it is important to understand what is normal for you, what changes you should expect, and when to speak with your doctor.

¹ Distribution shown using a quantile dot plot with 100 quantiles rounded to the nearest day. The point at 0 days indicates that 1% of participants have a median cycle length difference of 0 days.

Menstrual cycle length and variability: a visual explanation

Part 3

What is an irregular cycle?

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