How the Body Works: The Menstrual Cycle

Controlled by hormones, the average menstrual cycle is twenty-eight days. From about day 5 pituitary follicle-stimulating hormone, or FSH, promotes the growth of a follicle in the ovary. Follicular cells produce estrogen, which builds up the endometrium, the uterine lining, in readiness for implantation of the fertilized egg. In midcycle, a surge of pituitary luteinizing hormone, or LH, causes ovulation. The corpus luteum secretes progesterone, which reinforces estrogen and builds up the endometrium to its maximum thickness. If the egg is not fertilized, the corpus luteum degenerates in the last few days of the cycle and the lining is shed during menstruation, which marks the start of a new cycle. If fertilization occurs, about day 14 the fertilized egg secretes human chorionic gonadotrophin, or HCG. This maintains the corpus luteum and its output of progesterone and estrogen so that the endometrium is maintained during pregnancy and menstruation does not occur. The endometrium then nourishes the fertilized egg.