Menstrual cycle length and variability: a visual explanation

Study Update, September 2023

Part 4

What determines cycle characteristics?

In Parts 2 and 3 we looked at cycle length and cycle length variability across the participants in the AWHS. In Part 2, we learned that not everyone has 28-day cycles. In fact, 87% of participants in the study have a median cycle length between 24 and 38 days. In Part 3, we saw that most people's cycles aren't all exactly the same length. It's very common to have cycles vary by up to 5 days from one to the next. If you read the extra content in each section, you also learned that cycle length and variability change throughout a person's life.

In this part, we're going to explain these different kinds of variation in cycle characteristics by discussing ovulation and ovarian aging. You might have heard about ovulation mostly in the context of fertility and pregnancy. These concepts are very important if you're interested in conception now or in the future, but there are also good reasons to understand these processes and how they impact cycle length in general.

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.

In Part 1, we defined a cycle as the days from the first day of one period to the day before the next period starts.

Graphic: A calendar shows this month and next month with days arranged in a grid, as shown in part 1. Each day is shown with a grey square marked with the date. Today’s date, Thursday, March 7, and the following five days are marked with a red circle to indicate bleeding. The days in the calendar before today are faded. The next period is also shown with six red circles on six consecutive days, starting on Friday, April 5, 29 days from today. The days including and following the start of the next period are also faded so that only the current cycle is in focus.

Remember that ovulation is when an egg is released from the ovary, and it typically happens about 2 weeks before the start of the next menstrual period, assuming the egg is not fertilized.

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

Let's unwind this cycle again to look at what's going on in more detail.

Graphic: The squares from the cycle starting today shrink and are lined up in one long row of small squares. The first 6 squares are red for bleeding days. Next there are 8 grey squares, followed by a purple square for the ovulation day, then another 13 grey squares.

Each cycle has two main phases, which are divided by ovulation. The first phase, from the first day of the period to ovulation, is called the follicular phase. The second phase, after ovulation has occurred until the start of the next period, is called the luteal phase.

Graphic: The squares from the first to the ovulation (purple) square are labelled ‘follicular phase’. The purple day is labelled ‘ovulation’ and the squares after the ovulation day are labelled ‘luteal phase’.

The luteal phase typically lasts between 10 and 15 days [1]. The follicular phase has much more variability cycle to cycle and across the lifespan.

The majority of the variation in a person's cycle length can be attributed to variation in the length of the follicular phase.

Graphic: An animation changes the length of the follicular phase to add between 0 and 3 additional days.

Each ovary contains thousands or even millions of eggs (also known as oocytes), depending on a person's age [2]. Each egg is contained in a follicle, which is a small sac of fluid [3]. During the follicular phase of each cycle, many follicles are recruited and start developing to release an egg. Eventually one follicle (or occasionally more) will mature and open to release an egg (ovulation). One month this process might take a little longer, and another it might be faster.

Graphic: A diagram showing the stages of the follicular phase appears above the line of squares. The diagram is a series of five circles representing the follicle. The leftmost circle is the smallest and is a dark purple circle with a small, off-center, darker purple circle inside it – this represents the egg. The next circle is the same but with a light purple circle around the egg representing the follicle. The next circle is similar, but the follicle is larger. The next one is larger again and shows a lighter purple region that represents the fluid inside the follicle. The final circle, above the ovulation day, shows the lighter purple follicle opening to release the dark purple egg.

The amount of time that this process takes varies naturally from cycle to cycle, due to factors including nutrition, physical activity, stress, and health status, but it is also dependent on the number of follicles you have. This number decreases throughout your lifetime, which is why we see variation in cycle length and variability with age.

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

Why do cycle length and variability change as you age?

If you have ovaries, you are born with all the eggs you will ever have, somewhere from half a million to a million eggs. Even before your first period, which typically happens between 8 and 16 years old, your egg count begins to decrease. By the time you start menstruating, you might have about half of the eggs you were born with [2].

Between a person's first period and their last period, there is a continual decline in egg count. This decline gets gradually faster as you get older [2]. By the time a person reaches menopause, 12 months after their last period [4], their ovaries may contain below 1,000 eggs.

Although the changes in egg count over time are significant, especially in your 30s and 40s, the corresponding changes in cycle characteristics are not usually easily recognizable. The first change you might notice is slightly shorter cycles, maybe 2 or 3 days shorter on average than they used to be [2]. This is reflected in the AWHS data, where the participants in their 40s had the shortest median cycle lengths, on average.

The most noticeable change is increasingly irregular cycles leading up to the last menstrual period. At this point, it is common to have longer and longer cycles, and much more variability. This stage is called the menopausal transition and it typically starts around the mid to late 40s, although its onset can vary widely between people.

What does this information mean for you?

One reason that you might hear about ovarian reserves is in the context of fertility. A person's peak fertility is generally considered to be during their 20s and early 30s, and it may be increasingly challenging to get pregnant as you get older. While this is generally true, each person's individual ovarian aging process is unique, and one person's fertility at a certain age may be very different from another's. If getting pregnant is something you hope for in your future, make sure to talk to your doctor, regardless of age, to set yourself on the healthiest course.

However, fertility is not the only reason to know about ovarian reserves and how ovarian aging affects your menstrual cycle.

As you age and your menstrual cycle changes, your risk level for different diseases also changes. For example, the risk of cardiovascular diseases increases as you enter menopause [5]. This is important to know, as this time is a window of opportunity for lifestyle changes that may decrease the risk of cardiovascular disease in the future.

There are also sometimes unpleasant, uncomfortable, and unexpected symptoms associated with ovarian aging and the menopausal transition. While they may be inevitable, these symptoms can be treated and managed. If you know when to expect changes, and are tracking your cycles and symptoms, you will be more informed when you consult your doctor about treatment strategies.

Conclusion

The AWHS is using cycle tracking data to understand menstrual health at an unprecedented scale. In this series, we have introduced you to some of the key concepts that researchers and doctors use to characterize menstrual cycles: cycle length and cycle length variability. In this final part of the series we explained why those characteristics change naturally during a person’s life.

If you are a participant in the AWHS, thank you for sharing your data and contributing to our understanding of the menstrual cycle. In future study updates you can learn more about ways that the AWHS team is using this understanding to inform screening and risk assessments for gynecologic health conditions, including PCOS (polycystic ovary syndrome), infertility and breast cancer.