Menstruation is the most visible phase of the cycle, but the process involves more than bleeding. The menstrual phase ends one hormonal cycle and begins the next.
These changes take place over the 3–7 days of a typical period.
Why you bleed
Menstrual bleeding occurs when the uterine lining (endometrium) sheds after it is no longer supported by progesterone.
The sequence is:
- After ovulation, the corpus luteum (the structure left behind by the ruptured follicle) produces progesterone
- Progesterone maintains the thickened, blood-rich endometrium and prepares it for potential implantation
- If no pregnancy occurs, the corpus luteum degenerates around day 26–28
- Progesterone drops sharply, and the endometrium loses its hormonal support
- Blood vessels in the lining constrict and then rupture, allowing the tissue to break down and leave the body
Menstrual fluid contains blood, endometrial tissue, cervical mucus, and vaginal secretions. Only about 36% of it is blood by volume.
How much blood loss is normal?
Average total blood loss during a period is 30–40 mL, or 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.
- Frequently passing blood clots larger than a 10p coin 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. See how nutrition changes across cycle phases for iron-aware eating strategies.
What causes period cramps
Menstrual cramps (dysmenorrhea) are caused by prostaglandins, hormone-like compounds released as endometrial tissue breaks down. For a detailed look at evidence on cramp relief, see period pain relief.
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.
Several details help explain the timing and treatment of cramps:
- 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) block prostaglandin synthesis, which is why they are more effective than paracetamol for period pain.
- Taking NSAIDs before cramps begin, or at the 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. Hormone levels are at their lowest point:
- Estrogen is low and beginning to rise slowly.
- Progesterone is at its nadir. The drop is what triggered menstruation.
- FSH begins to rise as the pituitary responds to low estrogen and starts recruiting new follicles.
- LH is low.
Although the menstrual phase can be physically uncomfortable, the hormonal "reset" often brings a sense of mental clarity compared with the late luteal phase. The progesterone withdrawal that caused PMS symptoms is complete, and the new hormonal cycle has begun.
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 the period, it has usually returned to the 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 and the parasympathetic nervous system regains dominance, HRV trends upward.
Resting heart rate
RHR gradually decreases from its luteal-phase peak 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
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, as prostaglandin-induced contractions can radiate to the lower back
- Digestive changes because prostaglandins affect smooth muscle throughout the body, including the GI tract; some people experience looser stools during their period
- Headaches because 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
How the phase fits into the cycle
Your period ends the previous hormonal cycle and begins the next one. The changes are visible in bleeding and symptoms, and may also appear in temperature, HRV, resting heart rate, and sleep data.
References
- Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. Endotext. 2018.
- Critchley HOD, et al. Menstruation: science and society. American Journal of Obstetrics and Gynecology. 2020;223(5):624-664.
- Fraser IS, et al. A detailed analysis of menstrual blood loss. Australian and New Zealand Journal of Obstetrics and Gynaecology. 2001;41(2):125-127.
- Hallberg L, et al. Menstrual blood loss — a population study. Acta Obstetricia et Gynecologica Scandinavica. 1966;45(3):320-351.
- Percy L, et al. Iron deficiency and iron deficiency anaemia in women. Best Practice & Research Clinical Obstetrics & Gynaecology. 2017;40:55-67.
- Dawood MY. Primary dysmenorrhea: advances in pathogenesis and management. Obstetrics & Gynecology. 2006;108(2):428-441.
- Marjoribanks J, et al. Nonsteroidal anti-inflammatory drugs for dysmenorrhoea. Cochrane Database of Systematic Reviews. 2015;(7):CD001751.
- Romans SE, et al. Mood and the menstrual cycle: a review of prospective data studies. Gender Medicine. 2012;9(5):361-384.
- Brar TK, et al. Effect of different phases of menstrual cycle on heart rate variability. Journal of Clinical and Diagnostic Research. 2015;9(10):CC01-CC04.
- Baker FC, Driver HS. Circadian rhythms, sleep, and the menstrual cycle. Sleep Medicine. 2007;8(6):613-622.
- Bernstein MT, et al. Gastrointestinal symptoms before and during menses in healthy women. BMC Women's Health. 2014;14:14.
- MacGregor EA. Menstrual migraine: therapeutic approaches. Therapeutic Advances in Neurological Disorders. 2009;2(5):327-336.