The Biology of Periods

The Biology of Periods by Dr Dawn Harper

Dr Dawn Harper is a GP and leading media medic

Do you know your corpus luteum from your pituitary gland? Incredibly 1 in 5 women – and over a third of men – don’t know where period blood comes from, with some thinking it comes from the bladder!(1) And if we don’t understand the basics of the menstrual cycle, then how are we to know what’s normal and what’s not when it comes to periods? My blog aims to bring to life the biology of a period.

During the menstrual cycle, the wall of the uterus thickens with tissue and an egg is released from the ovary, if the egg is not fertilised then the body will shed the tissue through the vagina. The menstrual cycle is controlled by hormones and occurs in four phases: menstruation, follicular phase, ovulation and luteal phase (see image below).

Uterus. Text: Female reproductive system

Menstruation: menstruation is when the thickened lining of the uterus (endometrium) leaves the body through the vagina. The lining of the uterus supplies a fertilised egg with nutrients, so if the egg isn’t fertilised, then the lining is no longer needed.

Follicular phase: beginning on the first day of menstruation, the pituitary gland (a hormone releasing gland located at the base of the brain) releases follicle stimulating hormones (FSH). These hormones cause follicles to develop in the ovaries, which bead on the surface and eventually one will burst to release an egg. The follicles also cause a rise in oestrogen levels and the lining of the uterus to thicken.

Ovulation: the rise in oestrogen causes the levels of FSH and the luteinising hormone (LH) to increase. This occurs mid-way through the cycle and triggers the egg to be released into the uterus for fertilisation.

Luteal phase: the follicle where the egg originated remains on the surface of the ovaries and forms a structure called corpus luteum, releasing the hormones progesterone and oestrogen. This maintains the thickened lining of the uterus for a fertilised egg to attach. But if the egg remains unfertilised then it will die and so too will the corpus luteum. The lining of the uterus is then expelled from the body and the cycle repeats.

On average, the menstrual cycle lasts 21-35 days and menstruation lasts 4-7 days(2). But if you’re bleeding for more than seven days and your period occurs more frequently, then you could be suffering from heavy periods(3) which can have a significant impact on quality of life. This is a treatable medical condition with several treatment options available (not just a hysterectomy!) so please visit your GP if you’re suffering. I see lots of women with menstrual problems, in fact, more than 1 in 5 women suffer from heavy periods so there’s absolutely nothing to be embarrassed about. You can find out more about visiting your GP to talk about heavy periods on the Your GP Visit page on the Wear White Again website.

Wear White Again is a Hologic campaign.


(1) Data on file: MISC-05658-GBR-EN Rev 001
(2) Wear White Again, Talking Heavy Periods Guide MISC-05360-GBR-EN Rev 001. Last accessed July 2019, available at:
(3) Royal College of Obstetricians and Gynaecologists. National Heavy Menstrual Bleeding Audit (May 2011). Last accessed July 2019, available

SOM-00748-GBR-EN Rev 001

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