During the secretory phase of the uterus, the hormone progesterone is produced by the ovaries. Progesterone (as well as estrogen) is secreted by the corpus luteum, (which means yellow body), which develops from the Graafian follicle. Progesterone secreted by the corpus luteum stimulates the further build-up of the cells in the endometrium of the uterus. Progesterone also stimulates the glands in the uterus to secrete substances that maintain the endometrium and keep it from breaking down. For this reason, this phase of the menstrual cycle is called the secretory phase.
The presence of estrogen and progesterone in the blood inhibit the production of FSH and LH from the pituitary, and the levels of FSH and LH begin to fall.
If the egg that has been ovulated into the Fallopian tube is fertilized by spermatozoa, the developing zygote implants in the thickened endometrium of the uterus approximately seven days after ovulation. This stimulates the endometrium to secrete a human chorionic gonadotropic hormone (HCG). HCG maintains the corpus luteum in the ovary, so that it continues to secrete progesterone. HCG is secreted throughout pregnancy and keeps blood progesterone levels high, so that the endometrium continues to thicken, eventually forming the placenta. Without a high level of progesterone, the endometrium begins to break down. In a pregnancy, the breakdown of the endometrium would result in a miscarriage.
If fertilization does not occur, the corpus luteum shrinks and blood progesterone levels drop, at about day 22 in a 28 day cycle. Without progesterone, the endometrium degenerates, and is expelled through the cervix and out through the vagina.