Over your menstrual cycle, a membrane lining – made up of mucus, tissue, blood and other biological matter – forms in your uterus to prepare for pregnancy.
About mid-way through your cycle, one of your two ovaries will release an egg, called an ovum. If this ovum becomes fertilised by a sperm, the mucous membrane remains and a baby begins to develop.
However, in most cases…
The ovum won’t be fertilised so after a couple of days, it deteriorates along with the lining, which breaks down and is released by your body through your vagina – and that’s a period.