Starting from puberty, a woman ovaries and uterus perform a regular series of processes every 28 days.

• Day 1-5: Blood and tissue from the uterus pass out the vagina. This is called menstruation.

• Day 6-14: The lining of the uterus thickens to receive an egg from ovaries. In the ovaries, an egg starts to mature.

• Day 14-28: Around day 14, an egg is released from an ovary (ovulation). It travels down a fallopian tube to the uterus. If the egg is not fertilised, it leaves the body with the extra blood and tissue from the lining of the uterus. Then the cycle starts again.

An egg cell matures the ovaries every 28 days. The menstrual cycle begins to stop for women between 45 and 55 years old.