In girl with puberty, the ovaries start releasing matured eggs. There are two ovaries which are situated inside the abdominal cavity.
They produce one mature ovum each after every four weeks. The proces which a mature ovum is released from an ovary is called ovulation.
During period, the wall of the uterus becomes thick and this is a preparation to receive the fertilized egg if it so occurs. But if fertilization does not occur, the released egg, and the lining of the uterus along with its blood vessels are shed off. This causes bleeding called menstrual flow.
The bleeding in girls or women's due to break down of the thickened inner wall of uterus and blood vessels is called menstruation. The menstrual flow lasts for 4-7 days. Menstruation occurs once in about 28-30 days. The cycle changes in ovaries and uterus every twenty eight days by the menstrual flow is called menstrual cycle.
At 45 to 40 or 50 years of age, the menstrual cycle stops. The stopper menstrual flow and the other associated events in the body of a female is called menopause.