What Causes Menstrual Bleeding?
What causes menstrual bleeding is a fall in estrogen and progesterone. When no pregnancy happens, the uterine lining is no longer needed, so it breaks down and is shed as your period.
This guide covers the hormone steps, what menstrual blood contains and what changes flow. See normal cycle length and phases first, or track yours with the Period Calculator.
What Actually Causes Menstrual Bleeding?
Each cycle, your body builds up the endometrium, the lining of the uterus, in case of pregnancy. When none happens, hormone levels drop and the lining breaks down and leaves the body. That is your period. It is not a malfunction. It is the expected result of a cycle without pregnancy. Read more in the normal menstrual cycle guide.
The Hormonal Trigger Behind Your Period
After ovulation, progesterone keeps the lining thick in case a fertilized egg implants. If it does not, the corpus luteum (the structure left after the egg is released) breaks down and estrogen and progesterone fall sharply. Without progesterone, the blood vessels supplying the lining tighten and the tissue detaches. Bleeding then lasts about 3 to 7 days. The table shows the whole sequence.
| Stage | Hormone change | Effect on the lining |
|---|---|---|
| Early cycle | FSH helps an egg mature; estrogen rises | Lining starts to rebuild |
| Mid-cycle | LH surge triggers ovulation | Lining continues to thicken |
| After ovulation | Progesterone rises | Lining is held thick and ready |
| No pregnancy | Estrogen and progesterone fall; prostaglandins rise | Lining breaks down and is shed (your period) |
This also explains why stress, illness or thyroid problems can change when and how you bleed. See the female reproductive system guide for the organs involved.
Endometrial Physiology: What Is Shed and What Regrows
The endometrium has two layers. The upper functional layer builds up each cycle and is shed during a period. The deeper basal layer stays in place and regrows a new functional layer.
- Menstrual phase: the functional layer breaks down and bleeding occurs.
- Proliferative phase: rising estrogen rebuilds the lining and its blood vessels.
- Secretory phase: progesterone after ovulation makes the lining ready for a possible embryo.
Spiral arteries supply the functional layer. When progesterone falls, they tighten, the tissue loses blood supply and bleeding starts.
Step by Step: How the Uterine Lining Sheds
- Progesterone falls, so the small spiral arteries in the lining tighten.
- Blood supply to the upper lining drops, and the tissue starts to break down.
- Prostaglandins help the tissue detach, and the uterus contracts gently to move flow out through the cervix.
- Repair begins within days, and the blood vessels seal. The lining heals without scarring, so bleeding stops by itself.
Those contractions are why many people feel cramps. Our guide to period cramps explains why the pain happens and what can ease it.
The Science Behind Menstrual Bleeding
At tissue level, a period is a controlled, inflammation-like process, not simple leakage. The key steps are:
| Step | What happens | Why it matters |
|---|---|---|
| Progesterone withdrawal | Progesterone falls when no pregnancy occurs | Removes the signal that was holding the lining stable |
| Vessel tightening | Prostaglandins narrow the spiral arteries | Less oxygen reaches the upper lining, so it starts to break down |
| Tissue breakdown | Enzymes called matrix metalloproteinases break down the lining and immune cells enter the tissue | Lets the functional layer detach cleanly |
| Bleeding | Blood vessels in the detaching lining open and release blood | Flow is mixed with tissue and mucus |
| Limited clotting | Fibrinolytic enzymes such as plasmin break clots down | Menstrual blood often stays fluid; clots appear when flow outpaces this |
| Repair | Estrogen rises and the surface heals while bleeding is still ongoing | Bleeding stops without scarring |
These steps are why heavy periods can have different causes. Problems with hormones, clotting, blood vessels or tissue repair can each change how much you bleed.
Normal vs Abnormal Menstrual Bleeding: Duration and Volume
| Feature | Typical | Worth checking |
|---|---|---|
| Duration | 3 to 7 days; first two days often heaviest | Longer than 7 days |
| Total blood loss | About 30 to 40 mL, only a few tablespoons | More than about 80 mL (heavy menstrual bleeding) |
| Protection | Changed every few hours | Soaked through every hour for several hours |
| Clots | Small, smaller than a coin | Larger than a quarter, or frequent with heavy flow |
| Timing | Bleeding only during the period | Bleeding between periods |
| Effect on life | Manageable | Severe pain, tiredness, dizziness or disrupted daily life |
See how long a period lasts for typical ranges and the period blood color guide for what shades mean.
A Typical Period, Day by Day
| Days | Flow | What to expect |
|---|---|---|
| Days 1 to 2 | Heaviest | Cramps most likely; bright or dark red |
| Days 3 to 4 | Lightening | Most of the lining has already shed |
| Days 5 to 7 | Light or spotting | Sometimes pink or brown before it stops |
If your pattern is clearly different, for example very heavy on day 4 or lasting 10 days, note it and mention it to a doctor. The Period Calculator can store your start dates so you can compare cycles.
What’s Actually in Menstrual Blood
Menstrual flow is not just blood. It is a mix of:
- Blood: from the small vessels in the endometrial lining.
- Shed endometrial tissue: the uterine lining itself, broken down as it detaches.
- Cervical and vaginal mucus: which changes in texture across your cycle.
Blood that stays longer in the uterus has more time to oxidize. That is why flow is often brighter red on heavy days and darker or brownish at the start or end.
What Affects How Heavy or Light Your Flow Is
Flow depends mainly on how thick the lining became, which hormone levels control. Other factors:
- Age and life stage (flow often shifts after a first period and around perimenopause)
- Stress and significant lifestyle changes
- Hormonal birth control, which typically thins the lining and reduces flow
- Underlying conditions such as fibroids, thyroid issues or PCOS
Some cycle-to-cycle variation is normal. A sudden or ongoing change from your usual pattern is what matters. See common causes of menstrual cycle irregularities.
Menstrual Bleeding Through Different Life Stages
| Stage | What to expect |
|---|---|
| First years of periods | Often irregular because ovulation is not yet settled |
| Twenties and thirties | Usually the most predictable |
| After childbirth | First weeks of bleeding are lochia, not a true period; periods return at different times depending on breastfeeding |
| Perimenopause | Fluctuating hormones can make flow heavier, lighter, shorter or further apart |
Teenagers can start with the normal menstrual cycle guide.
Clinical Insights: Medical Causes of Heavy or Unusual Bleeding
Heavier or longer bleeding often has a cause that can be found and treated. Clinicians group causes as structural (polyps, adenomyosis, fibroids, cancer) or non-structural (clotting disorders, ovulation problems, medicines). Doctors commonly look at:
- Fibroids and polyps: non-cancerous growths that can increase flow or cause bleeding between periods.
- Adenomyosis and endometriosis: lining-type tissue growing in the wrong place, often with painful, heavy periods.
- Hormone imbalances: thyroid disease and PCOS can disturb ovulation and thicken the lining; see thyroid and the menstrual cycle.
- Bleeding or clotting disorders: sometimes first noticed as lifelong heavy periods.
- Medicines and devices: blood thinners, some anti-inflammatories and copper coils can raise flow.
Heavy bleeding can cause iron deficiency, so tiredness, dizziness or breathlessness with heavy periods should be checked. Treatment depends on the cause. The ACOG guide to heavy menstrual bleeding outlines the options.
Birth Control, Hormones and Your Bleeding
| Method | Usual effect on bleeding |
|---|---|
| Combined pill, patch or ring | Lighter, shorter, more predictable; a withdrawal bleed in the pill-free week, not a natural period |
| Hormonal coil or implant | Lighter, irregular or no bleeding |
| Copper coil | Often heavier periods at first |
Irregular spotting is common in the first three to six months. If bleeding is heavy, painful or does not settle, speak with a doctor before stopping or changing a method. Delaying a period for travel or events should also be discussed with a healthcare professional.
Common Myths About Period Bleeding
- “Period blood is dirty blood.” It is not. It is blood, lining tissue and mucus.
- “A heavier period means something is wrong.” Not always. It is the change from your own normal that matters.
- “Bleeding always means you are not fertile.” A regular period usually suggests ovulation is happening, but does not confirm it.
Is It Really a Period? Other Reasons for Vaginal Bleeding
- Ovulation spotting: light spotting mid-cycle.
- Breakthrough bleeding: common on hormonal contraception, especially in the first months.
- Implantation bleeding: a small amount in early pregnancy that can look like a short period.
If a bleed is much lighter, shorter or differently timed than usual and pregnancy is possible, take a pregnancy test. For unpredictable cycles, read PCOS and ovulation.
How to Track Your Bleeding Pattern
- Note the start date, length and heaviness of each period.
- Log start dates in the Period Calculator to estimate your next bleed.
- Read the early signs that a period is near, or predict your next period date by hand.
- Bring these notes to an appointment if anything seems off.
When Menstrual Bleeding Needs Medical Attention
Occasional variation in flow, color or clotting is rarely a concern. According to the NHS, see a doctor if bleeding becomes unusually heavy or comes with other symptoms.
Talk to a doctor if you notice:
- Bleeding that lasts longer than 7 days
- Soaking through a pad or tampon every hour for several hours in a row
- Clots larger than a quarter
- Bleeding or spotting between periods
- Severe pain that disrupts daily life
What We Know and Still Do Not Know About Menstruation
Well established: hormones fall, the lining loses blood supply and sheds, and repair then stops the bleeding. Still debated or not fully explained:
- Why humans menstruate at all, since few mammals do. One leading idea is that the lining prepares itself for pregnancy before an embryo arrives, so it must be shed when none implants.
- Why flow differs so much between people and between cycles.
- Why some people have heavy bleeding with no clear cause found.
Frequently Asked Questions
What causes menstrual bleeding?
What is menstrual blood actually made of?
Why does period blood sometimes look darker or have clots?
Is it normal for period flow to change from cycle to cycle?
Why does menstrual bleeding stop on its own?
Can you have a period without ovulating?
Is it normal to pass clots during a period?
Does stress change menstrual bleeding?
What hormones cause a period to start?
Is withdrawal bleeding the same as a period?
Disclaimer: This article is for information only and is not medical advice. If you have concerns about your bleeding, consult a healthcare provider.