Menstrual cycle length and variability: a visual explanation

Study Update, September 2023

A person's menstrual cycle length and regularity are key indicators of their overall health. The Apple Women's Health Study (AWHS ) is helping us characterize menstrual cycles throughout people's lives, by enrolling study participants who provide their permission for us to collect and analyze data on a larger scale than ever before. This understanding can help people prepare for their menstrual cycles and help patients and clinicians personalize their healthcare.

Many people are taught that there is a narrow range of "typical" menstrual cycle characteristics, but in reality there can be a wide range of cycle characteristics that are perfectly healthy, both between people and for one person over the course of their lifetime. Understanding your own cycle characteristics, and the range of healthy cycle characteristics in the population, can also help you figure out when it might be time to talk to your doctor.

In this series, we aim to help you understand the key concepts at the heart of the study, cycle length and variability, and how they vary between people in the study. This series is broken up into four parts. We recommend reading the parts in order, but you can navigate to any part at any time.

In Part 1: What is a menstrual cycle?, we define and explain key characteristics of a menstrual cycle. In Part 2: How long is a typical cycle?, we explore the range of cycle lengths of people from the study. In Part 3: What is an irregular cycle?, we look at how much a person's cycle varies from month to month. Finally, in Part 4: What determines cycle characteristics?, we discuss why we see variation in cycle characteristics through a person's life, and why it's important for everyone to understand these changes so that we can better prepare for them.

Part 1

What is a menstrual cycle?

First, we need to define some key concepts that are necessary for talking about cycle characteristics. Let's take it from the start: what is a cycle?

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.

Today is Thursday, March 7.

Graphic: A calendar shows this month and next month with the days of each month organized in a grid. Each day is shown with a grey square marked with the date. Each month grid is labelled with the month name. Today's date Thursday, March 7 is highlighted in black.

Imagine your period starts today.

Graphic: Today’s date is a grey square with a red circle on it indicating bleeding. The days in the calendar prior to today have been faded.

It might last around 6 days.
red circle is a bleeding day.

Graphic: Today and the five days following today are shown with a red circle to indicate a six day period starting today.

If you have average cycle lengths, you would then expect your next period to come about a month from now.

Graphic: The next period is also shown with six red circles on six consecutive days. The starting day for the next period is initially Friday, April 5, 29 days from today, but it animates changing between Tuesday, April 2 26 days from today, Thursday, April 4 28 days, Friday, April 5 29 days, Sunday, April 7 31 days, Monday, April 8 32 days, and Tuesday, April 9 33 days from today.

So let's say your next period comes on Friday, April 5.

Graphic: The next period is shown starting on Friday, April 5, 29 days from today.

A cycle starts on the day your period begins (noted by full-flow bleeding), and ends on the day before your next period starts. The cycle shown here is 29 days long.

Graphic: The days including and following the start of the next period are faded. Only the days from today to the day before the first day of the next period, inclusive, (one cycle) are in focus.

Cycles are dictated by hormones that control ovulation, which is when an egg is released from the ovary. It usually happens around 14 days before the start of the next period.
purple rounded square is the day of ovulation.

Graphic: The ovulation day is highlighted in purple on Friday, March 22, 14 days before the start of the next period.

We can unwind the cycle to see the cycle length more clearly. But looking at only one cycle doesn't tell us much about a person's typical cycle characteristics.

Graphic: The days from today to the day before the start of the next period, inclusive, shrink and unwind into one long line of small squares. The first six are red squares, indicating bleeding days, and the remaining 21 squares are grey. Everything else, including other days and the calendar month labels, disappears.

To learn more about someone's cycle characteristics, we need to line up lots of cycles over many months.

Graphic: Four more rows of small squares fly in from above to create a stack of five cycles, where each cycle is a line of small squares. Each row starts with a series of red squares, indicating period length, and then is followed by a number of grey squares so that the total number of squares including red and grey is equal to the cycle length. 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.

People participating in the Apple Women's Health Study join the Study through the Apple Research app and sign the Study informed consent form, after which they can choose to share the cycles that they have tracked through the Apple Health app with researchers.

In this series, we are reporting findings from analyses of over 1 million cycles recorded by almost 75,000 people. Everyone in the analysis has recorded at least three cycles. If you are using any kind of hormones, for example, a contraceptive pill or hormonal IUD, your cycles are influenced by those hormones, and they may look different from those of someone not using hormones. For this analysis, we only looked at cycles from people who are not using hormones.

In this study update, we won't be talking about period length, which is the number of bleeding days. Instead, we're going to focus on cycle length. As we see here, most people's cycles aren't exactly the same length every month. We'll dive more into this next.

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

In this first part, we've introduced two core concepts: cycle length and ovulation. Read on to Part 2 to learn more about cycle length in the Apple Women's Health Study.

Menstrual cycle length and variability: a visual explanation

Part 1

What is a menstrual cycle?

Resources