Menstrual cycle.

1. The hypothalamus releases GnRH (Gonadotropin-Releasing Hormone).

2. The pituitary secretes FSH and LH.

3. The FSH and LH stimulate follicle growth.

4. The follicles start to make estradiol. Several follicles begin to grow with each cycle, but usually only one matures.

5. Around day 12 estradiol levels rise steeply.

6. Rising estradiol stimulates a LH surge by positive feedback.

7. The LH surge triggers ovulation: the follicle ruptures, releasing the secondary oocyte.

8. The follicle left in the ovary forms the corpus luteum, which secretes progesterone and estradiol.

9. Rising progesterone and estradiol levels stimulate thickening of the uterine wall, or endometrium.

10. If pregnancy occurs, the corpus luteum continues secreting progesterone and estradiol to maintain the endometrium; otherwise it disintegrates and hormone levels drop, resulting in the loss of endometrial tissue as menstrual flow.

This continues for about 450 cycles, then women enter menopause.