Menstruation is a woman’s monthly normal vaginal bleeding, often called the period. When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which stay inactive until puberty begins. There isn’t one right age for a girl to get her period but most girls get their first period about 2 years after their breasts start to develop. when they’re between age 10 and 15 but it’s different, every girl’s body has its own schedule. two structures in the brain, the pituitary gland, and the hypothalamus produce complex hormones that stimulate the ovaries to make female sex hormones start and control The menstrual cycle. and it will continue to about age 51 . usually lasting from three to 8 days. The menstrual cycle includes several phases. The first phase of the menstrual cycle begins on the first day of bleeding and lasts for about 14 days. The pituitary gland (located at the base of the brain) releases a hormone called FSH. This hormone causes one fluid-filled “follicles contain a mature egg to develop. The maturing follicle produces the hormone called estrogen. which increases over and peaks in about day 12 and causes The lining of the uterus (endometrium) to become thicker and more enriched with blood vessels.in fact, the women’s body prepares to get pregnant.