In Part 1, we explained how we define cycle length. In this part, we're going to take a deeper
dive into what a "typical" cycle length looks like for people participating in the Apple Women's
Health Study.
O
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.
You might have heard that a typical menstrual cycle is 28 days long. But, there is a lot
of variation in cycle length, both between people, and for one person from one month to
another.
Graphic: The graphic shows 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 already read Part 1, this is the same visual representation from the
end of that part.
Let's zoom in to the ends of the cycles so we can compare their lengths more easily.
Graphic: The graphic 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.
This person's cycles range between 24 and 30 days.
The last square in each row is highlighted blue.
Let's line up these cycle lengths.
Graphic: The last square in each row and the number (length of the cycle) from each
label animates down onto an x-axis. The x-axis is labelled Cycle Length (Days) and the
values on the x-axis are 24, 27, 28, 29, 30. Above each of these labels is a single blue
square.
We can use the median as a measure of a person's typical cycle length. This person has
recorded five cycles, so their median cycle length is 28
days, the middle value of the five cycle lengths.
Graphic: The four squares, except the median (middle) square at 28 days, fade to a
lighter blue
That's right around what you might have heard as a typical cycle length.
Graphic: The squares move to the center and stack under the middle square at 28 days,
which becomes a circle. The axis labels except for 28 disappear.
But many people have cycles that are longer or shorter than 28 days.
In Health app, the shortest a cycle can be is 10 days. If your period stops and starts
again within 10 days, that's considered the same period.
Graphic: The axis shifts and the labels 10, 20, and 30 days are shown. A dashed vertical
line highlights the end of the x-axis at 10 days. The x-axis ends sharply at the 10 day
label on the left, and fades off to the right of the 30 day label.
In terms of longer cycles, 90 days is the longest cycle that we consider. Clinical
guidelines suggest seeing a doctor if you ever go more than 90 days without a period.
Graphic: The axis shifts to the right and the labels for 40, 50, 60, 70, 80, and 90 days
appear. The vertical line at 10 days disappears and one is shown at 90 days. The x-axis
ends sharply at 10 days on the left and 90 days on the right.
We have defined a "short" cycle as less than 24 days, and a "long" cycle as longer than
38 days. Most people have a median cycle length between 24 and 38 days, but not
everyone!
Graphic: The axis zooms in to 20 to 40 days, with labels at increments of 4 days. The
axis fades out on both ends. A blue highlighted region appears between 24 and 38 days.
Let's make up 100 imaginary people who are representative of the participants in the
AWHS. How many people do you think would
have a median cycle length between 24 and 38 days?
Graphic: 100 circles in a grid appear above the blue highlighted region. The leftmost 50
circles are blue and the rightmost 50 circles are grey. As you adjust
the slider, the number of circles highlighted blue changes to always reflect the slider value.
blue circleis 1 person
You guessed 50.
It's actually much more! 87 out of 100 people would have a median cycle length between
24 and 38 days.
Graphic: 87 of the 100 circles drop down from the grid to the x-axis, creating a
quantile dot plot between 24 and 38 days. These dots form a roughly bell-curved shape
with a large spike at 28 days, and a slightly heavier tail to the right. 13 dots remain
in the grid. The quantile dot plot has a stack of dots at each 1 day bin on the x-axis.
At 24 days there are 3 dots. At 25 days there are 4 dots. At 26 days there are 10 dots.
At 27 days there are 10 dots. At 28 days there are 19 dots. At 29 days there are 10
dots. At 30 days there are 9 dots. At 31 days there are 5 dots. At 32 days there are 6
dots. At 33 days there are 3 dots. At 34 days there are 3 dots. At 35 days there is 1
dot. At 36 days there are 2 dots. At 37 days there is 1 dot. At 38 days there is 1 dot.
The remaining 13 people would have median cycle lengths
outside of this range.
Graphic: All 100 dots are in the quantile dot plot.
Three people would have a median cycle length shorter than 24 days.
Graphic: The axis shifts to the right so that the visible region of the x-axis ranges
from 10 to approximately 40, and the labels are at 10, 20 and 30 days. There is a blue
highlighted region from 10 to 23 days, and there are three dots in the highlighted area,
one at 19 days, one at 22 days, and one at 23 days.
Ten people would have a median cycle length longer than 38 days.
Graphic: The axis shifts to the left so that the visible region of the x-axis ranges
from approximately 35 to 90 days. There is a blue highlighted region from 38 to 90 days,
and there are 10 dots in this region. There is 1 dot at 39 days, one dot at 40 days, 4
dots between 41 and 50 days, 2 dots between 50 and 60 days, one between 60 and 70, and
one dot at 90 days.
When we look at menstrual cycles from the tens of thousands of people participating in
the AWHS, we can see that plenty of
people have cycles that aren't exactly 28 days.1
Graphic: The plot shifts so that the visible x-axis ranges from around 20 to 50 days.
The axis ticks range from 20 to 44 by increments of 4. The dots form a roughly
bell-curved shape with a large spike at 28 days, and a slightly heavier tail to the
right. The quantile dot plot has a stack of dots at each 1 day bin on the x-axis. At 24
days there are 3 dots. At 25 days there are 4 dots. At 26 days there are 10 dots. At 27
days there are 10 dots. At 28 days there are 19 dots. At 29 days there are 10 dots. At
30 days there are 9 dots. At 31 days there are 5 dots. At 32 days there are 6 dots. At
33 days there are 3 dots. At 34 days there are 3 dots. At 35 days there is 1 dot. At 36
days there are 2 dots. At 37 days there is 1 dot. At 38 days there is 1 dot.
This is the end of the section with the visualization graphic.
In this part, we've looked at how typical cycle length varies in the population of participants
in the Apple Women's Health Study. It is useful to know your typical cycle length, so that you
can keep track of your health and notice if something is changing. The typical cycle length is
only one way of characterizing a person's menstrual cycle. Another important characteristic is regularity, or how much the length of a person's cycle varies from month to month. Read on to Part 3 to
learn more about cycle regularity.
1
Distribution shown using a
quantile dot plot
with 100 quantiles rounded to the nearest day. The point at 19 days indicates that 1% of participants have a median cycle length between 10
(the minimum) and 19 days.