The hypothalamus first receives information from the environment (such as season and availability of a mate) and then releases reproductive hormones to the pituitary gland, which controls (under the guidance of the hypothalamus) many endocrine functions. When low levels of estrogen are present, the menstrual cycle begins, triggered by negative feedback.

The hypothalamus releases GnRH which triggers the pituitary gland to release FSH and LH (luteinizing hormone). FSH stimulates the egg’s development and estrogen production. Positive feedback occurs as the anterior pituitary releases additional LH. The large rise in LH which comes from the pituitary and anterior pituitary causes ovulation (release of the egg).

Post-ovulation brings the follicle to keep producing estrogen and progesterone. The uterus lining thickens, and the follicle become the corpus luteum. Negative feedback due to progesterone cause a falloff in LH and FSH. If the egg is fertilized, the developing cells release HCG which maintains the endometrium. Otherwise the ovulating egg leaves the body with the corpus luteum and the cycle repeats.