Puberty is the name given to sexual maturity, which normally occurs between ages 10-16. During this time for girls, breasts develop, the hips widen, and menstrual periods begin. For boys, the voice deepens, facial hair grows, and the testes and penis enlarge. The opening to the uterus is called the cervix. If an egg is not fertilized, the thick spongy lining of the uterus sheds, in a process called menstruation. On average, the menstrual cycle lasts 28 days.