Each month, the female human body goes through a series of complex events as part of the menstrual cycle.
At the beginning of your menstrual cycle, your oestrogen and progesterone drop if there is no pregnancy, causing the menses to start. The pituitary gland start to make FSH and LH which stimulates the growth of a new follicle in the ovary.
One follicle becomes the dominant follicle, and it starts to produce oestrogen. The increasing amount of oestrogen makes the lining of the uterus thicken.
At about roughly day 12-13, the increasing oestrogen levels signals to the pituitary to create a surge in LH.
The heightened LH causes the dominant follicle to be released. Where the follicle used to be are some cells called the corpus luteum. When they detect that a follicle has been expelled, they start to produce progesterone.
The higher levels of progesterone get the lining ready in case of fertility, as it supplies the endometrium with nutrients. It also tells the pituitary to stop the production of LH and oestrogen.
Roughly 12-14 days after the egg was released, the corpus luteum dies. This causes both oestrogen and progesterone levels to drop if there is no fertility.
The drop in hormone signals the shedding of the endometrium and a period ensues.