What Happens During Your Period: The Menstrual Phase Explained

Menstruation is the most visible phase of the cycle, yet it's often the least understood beyond "you're bleeding." The menstrual phase is actually a precisely orchestrated biological reset — the end of one hormonal cycle and the beginning of the next.

Here's what's really happening inside your body during those 3–7 days.

Why you bleed

Menstrual bleeding occurs because the uterine lining (endometrium) sheds when it's no longer being supported by progesterone.

Here's the timeline:

1. After ovulation, the corpus luteum (the structure left behind by the ruptured follicle) produces progesterone
2. Progesterone maintains the thickened, blood-rich endometrium — preparing it for potential implantation
3. If no pregnancy occurs, the corpus luteum degenerates around day 26–28
4. Progesterone drops sharply → the endometrium loses its hormonal support
5. Blood vessels in the lining constrict, then rupture → tissue breaks down and is expelled

The resulting menstrual fluid is a mixture of blood, endometrial tissue, cervical mucus, and vaginal secretions. It's not just blood — only about 36% of menstrual fluid is actually blood by volume.

How much blood loss is normal?

The average total blood loss during a period is 30–40 mL — roughly 2–3 tablespoons. The clinical definition of heavy menstrual bleeding (menorrhagia) is more than 80 mL per cycle.

Practical gauges:

• Soaking through a pad or tampon in less than 2 hours suggests heavy bleeding
• Passing blood clots larger than a 10p coin frequently is worth discussing with a doctor
• Periods lasting more than 7 days are considered prolonged

Heavy periods are the leading cause of iron deficiency in premenopausal women. If your periods are consistently heavy, checking ferritin levels (not just hemoglobin) is advisable.

What causes period cramps

Menstrual cramps (dysmenorrhea) are caused by prostaglandins — hormone-like compounds released by the disintegrating endometrial tissue.

Prostaglandins serve a purpose: they cause the uterine muscle to contract, which helps expel the lining. But excessive prostaglandin production causes more intense contractions, reduced blood flow to the uterus, and pain.

Key facts:

• Prostaglandin levels are highest during the first 48 hours of menstruation — which is why day 1 and day 2 are typically the worst
• People with more painful periods have been shown to produce higher levels of prostaglandin F2α
• NSAIDs (ibuprofen, naproxen) work specifically by blocking prostaglandin synthesis — which is why they're more effective than paracetamol for period pain
• Taking NSAIDs before cramps begin (or at the very first sign) is more effective than waiting until pain is established

What your hormones are doing

Day 1 of your period is also day 1 of the follicular phase. Hormonally, this is the lowest point:

• Estrogen: Low and beginning to slowly rise
• Progesterone: At its nadir — the drop is what triggered menstruation
• FSH: Beginning to rise as the pituitary responds to low estrogen, starting the process of new follicle recruitment
• LH: Low

This hormonal "reset" is why the menstrual phase, while physically uncomfortable, often brings a sense of mental clarity compared to the late luteal phase. The progesterone withdrawal that caused PMS symptoms is complete, and the new hormonal cycle is beginning.

Biometric patterns during your period

If you're tracking with an Apple Watch:

Wrist temperature

Temperature typically begins to drop during menstruation as progesterone clears the system. By the end of your period, you've usually returned to your follicular-phase baseline. This is one of the most reliable biometric markers of menstrual phase timing.

HRV

HRV generally begins to recover during menstruation. As progesterone drops et the parasympathetic nervous system regains dominance, HRV trends upward.

Resting heart rate

RHR gradually decreases from its luteal-phase peak back toward follicular-phase baseline levels.

Sleep

Sleep quality often improves during the latter days of menstruation as core body temperature drops — making it easier to fall and stay asleep.

Common menstrual-phase symptoms

Beyond cramps and bleeding, other common symptoms during menstruation include:

• Fatigue — particularly in the first 1–2 days, driven by prostaglandin activity and, for some, blood loss
• Lower back pain — prostaglandin-induced contractions can radiate to the lower back
• Digestive changes — prostaglandins affect smooth muscle throughout the body, including the GI tract, which is why some people experience looser stools during their period
• Headaches — the sudden estrogen drop can trigger menstrual migraines in susceptible individuals

When to talk to a doctor

Most menstrual symptoms are normal variations, but some warrant medical evaluation:

• Periods that consistently last longer than 7 days
• Needing to change protection more than every 1–2 hours
• Period pain that doesn't respond to NSAIDs or prevents normal activity
• Cycles shorter than 21 days or longer than 35 days
• Sudden changes in your established pattern

The bottom line

Your period is a biological reset — the moment your body clears the previous cycle's hormonal slate and begins building toward the next ovulation. It's the most measurable phase of the cycle, both in what you experience and what your wearable data shows.

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References

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