The hypothalamus stimulates the pituitary gland to secrete FSH which causes the follicles to grow. Follicles secrete estrogen as they grow, stimulating the early steps in the build up of the uterine lining. Only one follicle continues developing and filling with fluid.
LH from the pituitary causes the cavity of the follicle to become filled with yellow tissue called corpus luteum; corpus luteum produces progesterone and estrogen, promoting further build up of the uterine lining.
Estrogen and progesterone from the corpus luteum cause the uterine lining to thicken, preparing for the embryo. If no embryo implants, the uterine lining breaks down. Unfertilized egg, blood, and tissue are expelled from the body via vagina.