What Causes Menstrual Bleeding?
Every month, your body prepares for a possible pregnancy by building up the lining of the uterus — and when that pregnancy doesn’t happen, your period is simply that lining being shed. This guide explains the hormonal process behind menstrual bleeding, what’s actually in the blood, and what can affect how heavy or light it is.
What Actually Causes Menstrual Bleeding?
Every menstrual cycle, your body prepares for a possible pregnancy by building up the endometrium — the lining of the uterus. When pregnancy doesn’t happen, that preparation is no longer needed, and hormone levels drop, signalling the lining to break down and leave the body. That’s your period. So what causes menstrual bleeding, in short? It isn’t a malfunction — it’s the natural, expected result of a cycle without a pregnancy.
The Hormonal Trigger Behind Your Period
After ovulation, progesterone rises to keep the endometrial lining thick and ready, in case a fertilised egg needs to implant. If implantation doesn’t occur, the corpus luteum (the structure left behind after the egg is released) breaks down, and both estrogen and progesterone levels fall sharply.
This hormonal drop is what causes menstrual bleeding to begin. Without progesterone maintaining it, the blood vessels supplying the lining constrict and then the tissue breaks down, detaches, and is shed through the vagina — a process that typically takes 3 to 7 days.
What’s Actually in Menstrual Blood
Despite the name, menstrual flow isn’t just blood, and knowing what causes menstrual bleeding also means knowing what it’s actually made of. It’s 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
This mix is also why colour and consistency can vary during a single period — blood that spends longer in the uterus has more time to oxidize, which is why flow is often brighter red on heavier days and darker or brownish at the very start or end.
What Affects How Heavy or Light Your Flow Is
Beyond the basic hormonal trigger, what causes menstrual bleeding to be heavier in one cycle versus lighter in another comes down to how thick the endometrial lining became before shedding, which is itself influenced by hormone levels. A few other everyday factors can also play a role:
- Age and life stage (flow patterns often shift after a first period and again 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 in flow is completely normal — it’s a significant, sudden, or ongoing change from your usual pattern that’s worth paying attention to. For more on cycles that fall outside a typical range, see our guide to common causes of menstrual cycle irregularities.
When Menstrual Bleeding Needs Medical Attention
Understanding what causes menstrual bleeding under normal circumstances also helps you recognize when something’s off. Occasional variation in flow, colour, or clotting is rarely a concern. According to the NHS, it’s worth seeing a doctor if bleeding becomes unusually heavy or is accompanied by 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 consecutive hours
- Clots larger than a quarter
- Bleeding or spotting between periods
- Severe pain that disrupts daily life
Frequently Asked Questions
A few more common questions about what causes menstrual bleeding, answered briefly.