Menstrual Phase

What Happens During the Menstrual Phase

A period is progesterone withdrawal made visible. Through the luteal phase, the corpus luteum keeps the endometrium thick and receptive. If no pregnancy starts, that structure breaks down, progesterone and estradiol drop sharply, and the endometrium can no longer be sustained without them, so it sheds.^[1] Researchers describe what follows as an inflammatory-like event: the spiral arterioles feeding the tissue constrict intensely, the outer layer breaks down and bleeds away, and repair starts from the surviving basal layer.^[2]

Where It Sits in the Cycle

Here's the honest wrinkle: physiologically, the menstrual phase isn't a separate phase at all. Day 1 of bleeding is day 1 of the follicular phase, so menstruation happens during that phase's opening days while FSH is already recruiting the next round of follicles.^[1] Trackers, including ours, still show "menstrual" as its own label because the bleed is the one part of the cycle you can see and log directly, and it anchors every other estimate the app makes.

How Long It Lasts in Studies

Published averages depend on who was studied and how. A pooled analysis of 3,324 cycles from 581 women without known fertility problems found bleeding averaged 6.2 days, with a standard deviation of 1.5 days.^[3] A study of 612,613 app-tracked cycles reported a shorter mean bleed of 4.0 days, also with a standard deviation of about 1.5 days.^[4] Those are study findings from two different populations and logging methods, not a range anyone's period is supposed to fit. For how the whole hormonal sequence connects bleed to bleed, see our hormone curve across your cycle guide.

How Go Go Gaia Tracks the Menstrual Phase

Log your flow with a 1-click entry each day you bleed, and Go Go Gaia marks those days as your menstrual phase, counting day 1 of bleeding as day 1 of your cycle. Your logged period starts anchor the phase labels for the rest of the cycle, so everything else you track, like mood, sleep, or BBT, charts against the right phase automatically.

What the Research Shows

Menstruation is now studied as a cycle of injury and repair, not just shedding. Reepithelialization starts while bleeding is still happening, rebuilding the endometrial surface piecemeal from stem and progenitor cells in the basal tissue.^[2]

Bleed length also varies within the same person. In the pooled cohort analysis, 11.6% of women had a gap of more than 3 days between their own longest and shortest menses.^[3]

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Is the menstrual phase part of the follicular phase?

Yes. Physiologically, day 1 of bleeding is day 1 of the follicular phase, so menstruation happens during the follicular phase's opening days.^[1] Trackers usually show it as its own phase anyway, because bleeding is the part of the cycle you can directly observe and log.

How long does menstrual bleeding usually last?

It depends on the study. A pooled analysis of 3,324 cycles from women without known fertility problems found bleeding averaged 6.2 days, while a study of over 600,000 app-tracked cycles reported a mean bleed of 4.0 days.^[3][4] Both found a standard deviation of about 1.5 days, so these are population averages, not targets.

What actually triggers a period to start?

Progesterone withdrawal. When the corpus luteum breaks down at the end of the luteal phase, progesterone and estradiol drop sharply, and the endometrium can't be sustained without them.^[1] The tissue breaks down and sheds, which is the bleeding you see.^[2]