The menstrual cycle is a monthly series of changes in a woman's reproductive system. It lasts about 28 days and is controlled by hormones, mainly oestrogen and progesterone, which are made in the ovaries.
The stages are:
- Days 1 to 5: menstruation. The lining of the uterus breaks down and is lost through the vagina.
- Days 5 to 14: the lining of the uterus repairs and thickens. Oestrogen causes this.
- Around day 14: ovulation. A mature egg is released from an ovary.
- Days 15 to 28: the lining is maintained, ready for a fertilised egg to implant. Progesterone maintains it.
If the egg is not fertilised, the level of progesterone falls, the lining breaks down and a new cycle begins with menstruation.
