Progesterone acts through its receptors. The luteinizing hormone (LH) stimulates the receptor expression and receptors begin to appear in the granulosa cells of the dominant follicle in the periovulatory period. Progesterone receptor expression varies in different phases of the menstrual cycle. Progesterone has an important role in mediating the slowing of GnRH pulses in the late luteal phase which causes a rise in FSH necessary for initiating the next cycle. Progesterone prepares and maintains the endometrium to allow implantation of the fertilized zygote. Progesterone is important in preventing maternal rejection of trophoblast by suppressing the maternal immunologic response to fetal antigens.