Women have two ovaries. The ovaries usually release one egg cell a month, in the middle of the menstrual cycle. This is called ovulation. Each ovary releases an egg cell every second month. When a girl is born, her ovaries already contain all the egg cells she will release during her entire life. At birth, her ovaries contain about one million egg cells, but when she starts to menstruate there are about 400,000 left. From the time of first menstruation to the end of the menopause, 400–500 egg cells mature and are released in order to be fertilised by a sperm cell, and a new human life is created.
The ovaries continue to mature and release egg cells until the woman reaches the menopause when she is 45–55 years old.