A period is triggered not by a hormone rising but by one crashing
The monthly cycle is orchestrated from the brain, which releases hormones in pulses that tell the ovaries when to ripen and release an egg. If no embryo implants, a temporary gland in the ovary shuts down, progesterone and oestrogen plunge, and that sudden drop is what makes the uterus shed its lining.
The menstrual cycle is really two coordinated cycles. The ovarian cycle governs the maturing and release of eggs along with the rhythmic output of oestrogen and progesterone. The uterine cycle prepares and maintains the womb's lining, the endometrium, to receive an embryo. Together they normally last 21 to 35 days, with a median of 28. The first period usually arrives around age 12, and cycles continue for roughly 30 to 45 years, ending at menopause, typically between 45 and 55. They tend to be less regular near the beginning and end of that span.
Control starts in the hypothalamus, which releases gonadotropin-releasing hormone. That prompts the pituitary gland just below to secrete follicle-stimulating hormone and luteinising hormone. Before puberty the signal trickles out steadily; afterwards it comes in large pulses whose rhythm and strength set how much of each pituitary hormone is made.
Day one is the first day of bleeding, which lasts about five days. During the follicular phase, rising oestrogen thickens the lining while several follicles in the ovary start to grow; usually just one becomes dominant and the rest wither. Oestrogen peaks around day 13. Roughly 10 to 12 hours after a surge of luteinising hormone, typically near day 14, the dominant follicle releases its egg, which survives for a day or less unless fertilised. The emptied follicle becomes the corpus luteum, pumping out progesterone to ready the lining for implantation.
If no embryo implants within about two weeks, the corpus luteum degenerates into a hormone-free remnant called the corpus albicans. Progesterone and oestrogen drop sharply and the lining breaks down. Many women experience premenstrual syndrome, with tender breasts or tiredness; a more severe form, premenstrual dysphoric disorder, affects about 3 to 8 percent. Hormonal contraception can alter the cycle.
Source: Menstrual cycle
