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The four phases of a menstrual cycle, in plain words

By Chris Ayers 7 min read

Most menstrual cycle phases explained online use a tidy 28-day circle with ovulation sitting at the top on day 14. The circle is not wrong, exactly. It is an average. The trouble is that averages are smooth and real cycles are not, and once you know which half of the cycle does the moving, a lot of confusing things start to make sense. This is the version I wish someone had handed me before I started building a cycle app.

The cycle in one sentence

The NHS puts it simply: the menstrual cycle is the time from the first day of your period to the day before your next period. Day 1 is the first day of bleeding. Everything else counts from there.

Cleveland Clinic gives the standard framing. The average cycle is 28 days, but a cycle can run from 21 days to about 35 and still be normal. Inside that span, there are four phases. In a 28-day example they fall roughly like this: menstruation from day 1, lasting about 3 to 7 days; the follicular phase from day 1 to about day 14; ovulation around day 14; and the luteal phase from about day 15 to day 28.

Notice that the first two overlap. Your period is the opening stretch of the follicular phase, not something separate from it. That trips people up.

Menstruation

Day 1 is the first day of your period. Over about 3 to 7 days the lining of the uterus, which built up during the previous cycle, leaves the body. Cleveland Clinic explains that estrogen and progesterone drop at the end of a cycle with no pregnancy, so both start this phase low. This is the stretch that research on exercise tends to single out, which I cover in the research overview and in lifting on your period.

The follicular phase

The follicular phase starts on day 1 too and runs until ovulation. The name comes from follicles in the ovaries, which grow during this phase. Cleveland Clinic says that around days 10 to 14 one of them forms a fully mature egg. Over the same stretch the level of the hormone estrogen rises, which causes the lining of the uterus to grow and thicken again.

In the 28-day example, this phase is about two weeks. In real life it is the phase that varies most, and I will come back to why that matters.

Ovulation

Ovulation is the release of the egg. Cleveland Clinic describes the trigger: a rise in luteinizing hormone, usually shortened to LH, causes the ovary to release its egg. In the textbook cycle this lands around day 14.

The NHS gives a more useful way to think about it. In most women, ovulation happens around 10 to 16 days before the next period. Read that carefully. It is counted backward from the next period, not forward from the last one. That is the key to the whole thing.

The luteal phase

After ovulation, the hormone progesterone rises. Cleveland Clinic’s wording is that it rises to help prepare the uterine lining. If no pregnancy happens, estrogen and progesterone drop, the lining sheds, and the next period starts. Back to day 1.

Why day 14 is an average, not a rule

Here is the part the circle hides. If the two halves of the cycle were equally fixed, ovulation would land in the same spot every month. They are not.

In 2019, Bull and colleagues published a study in npj Digital Medicine built on 612,613 cycles from 124,648 women who tracked them in an app. The mean cycle length was 29.3 days, already a little longer than the famous 28. More to the point, they measured the two halves separately. The follicular phase averaged 16.9 days, with a 95 percent range from 10 to 30 days. The luteal phase averaged 12.4 days, with a 95 percent range from 7 to 17 days.

Look at those ranges side by side. The first half of the cycle spans twenty days of normal variation. The second half spans ten. The normal range for the luteal phase is half as wide.

A much smaller and older study found the same shape. Fehring and colleagues followed 141 healthy women through 1,060 cycles in 2006. The mean cycle was 28.9 days, and 95 percent of cycles fell between 22 and 36 days. Within a single woman, cycle length varied by more than 7 days in 42.5 percent of participants. Their conclusion: “there is considerable normal variability,” and “the follicular phase contributes most to this variability.”

So when your cycle runs long one month, it is usually the first half that stretched. The egg was released later than usual, and the second half ran something close to its usual 12 or so days after that. Ovulation did not happen on “day 14.” It happened whenever the follicular phase finished, and the follicular phase does not keep a schedule.

This is also why the NHS counts ovulation backward from the next period. The gap after ovulation is the reliable one. The gap before it is not.

What this means in practice

Three things follow from the data above.

A cycle that is 25 days one month and 33 the next can be entirely normal. Fehring’s 42.5 percent figure says swings of more than a week between cycles are common in healthy women, not a sign something is off. If your cycles move outside the usual range or change pattern, that is a conversation for your doctor, and I have written up what the guidelines say about that in how long a normal cycle is.

Phase labels on a calendar are a guess until the cycle is over. An app that says “you are in your luteal phase” on day 16 is assuming ovulation already happened. Sometimes it has. Sometimes the follicular phase is still going.

And the second half of your cycle is the part you can plan around with more confidence, because it is the steadier half. If you notice a pattern that shows up in the week before your period, it will tend to land at about the same distance before your period each month, even when the cycle as a whole is longer or shorter.

I built Ferrabelle to lean on exactly this. The app starts from population averages like the ones above, conditioned on your age, and then replaces them with your own history as you log cycles. The phase forecast it shows is an estimate with a confidence label attached, not a verdict. That is the honest way to draw the circle.

This is general information, not medical advice. Talk to your doctor about your own health.

Ferrabelle is in a closed test on Google Play right now. If you want to see your own cycle laid out this way, you can join at ferrabelle.com/testers.

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