What actually happens during your period?
During menstruation, the lining of your uterus sheds and leaves the body through the vagina as period blood. It happens because no pregnancy started: the corpus luteum stops making progesterone, hormone levels fall, and the lining breaks down. The uterus contracts to push it out, which causes cramps. A period usually lasts 2 to 7 days.
Why we have periods
Every cycle, your body prepares for a possible pregnancy. Estrogen builds up the lining of the uterus (the endometrium), and after ovulation, progesterone makes it thick, soft and full of blood vessels, ready for a fertilized egg to implant. If no pregnancy starts, that lining isn't needed. A period is the body clearing it out so it can start fresh.
What happens, step by step
- Hormones fall: about 12 to 14 days after ovulation, the corpus luteum fades and progesterone and estrogen drop.
- Blood supply shrinks: small arteries in the lining tighten, and the outer layer starts to break down.
- Prostaglandins rise: these chemicals make the uterine muscle contract, squeezing the lining out. They're the main cause of cramps.
- Shedding: blood, tissue and mucus leave through the cervix and vagina over several days.
- Clotting control: the body uses natural anticoagulants so blood flows, but small clots are common on heavier days.
- Rebuilding starts: even before bleeding stops, a new follicular phase has begun and estrogen starts to rise again.
What's typical
| Feature | Typical range |
|---|---|
| Length of bleeding | 2 to 7 days |
| Blood lost per period | Usually about 30 to 40 ml (2 to 3 tablespoons); over 80 ml is considered heavy |
| Heaviest days | Usually the first 2 days |
| Color | Bright red, dark red, brown toward the end |
| Time between periods | 21 to 35 days in adults |
More detail in how long a period lasts, how much blood you lose and period blood colors.
What else your body is doing
Prostaglandins don't only act on the uterus. They can also affect the bowel, which is why loose stools are common on day 1 or 2, and they can cause headaches, nausea or an achy, flu-like feeling. Low estrogen and progesterone can leave you tired. On the bright side, many people feel PMS mood symptoms ease within a day or two of bleeding starting.
See a doctor if you soak a pad or tampon every 1 to 2 hours, pass clots bigger than about 2.5 cm (1 inch), bleed for more than 7 days, have pain that stops you doing normal things, or feel faint or breathless. Also get checked if periods stop for 3 months or more and you're not pregnant.
Frequently asked questions
Is period blood just blood?
No. It's a mix of blood, tissue from the uterine lining, mucus and vaginal fluid, which is why it can look thicker or contain small clots.
Why do I poop more on my period?
Prostaglandins that make the uterus contract can also speed up the bowel. See why periods make you poop.
Do you lose an egg in your period?
No. The egg from ovulation breaks down and is absorbed about a day after release, long before your period.
Why am I so tired during my period?
Low hormone levels, cramps, disrupted sleep and blood loss can all play a part. Heavy periods can also lead to low iron. See why you're tired on your period.
Sources
- Cleveland Clinic: Menstrual Cycle
- NHS: Periods
- ACOG: Heavy Menstrual Bleeding FAQ
- ACOG: Dysmenorrhea: Painful Periods FAQ
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
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