The menstrual cycle is controlled by two hormones released from the brain, specifically the pituitary gland which releases follicle stimulating hormone (FSH) and lutenising hormone (LH).
This causes an egg cell located in the female reproductive system to grow and mature.
In response, follicles release oestrogen and cause eggs to grow. This inhibits the production of FSH and increases the release of LH, eventually causing the release of the egg, otherwise known as ovulation.
Meanwhile the empty follicle releases progesterone to promote the uterus lining to develop, ready for a fertilised egg.
If the egg is fertilised, this equals pregnancy. However, if not fertilised, there is a drop in both oestrogen and progesterone and the uterus lining breaks down, which causes a period.
Then the process starts again!