A girl gets her period because she has a uterus, fallopian tubes and ovaries. During puberty, the small follicles in the ovaries begin to develop and the estrogen in the female body slowly rises. Estrogen promotes the proliferation of the endometrium and also stimulates the hypothalamus and pituitary gland to secrete follicle-stimulating hormone and luteinizing hormone. The hormones, in turn, stimulate the growth of follicles, which can cause the endometrium to change between the proliferative and secretory phases. The regulation of the mechanism in a month or so will lead to a decrease in hormone levels, and the endometrium will shed when it loses hormonal protection, causing bleeding and leading to menstruation. After menstruation, the small follicles in the ovaries start to develop again, and the cycle continues, so women will have their periods.