Menstrual Cycle

Flow Phase

Days 1-5. Marks the beginning of the cycle. Characterized by the shedding of the endometrium. Hormone level are at their lowest. Oxcytocin is high. 

Follicular Phases

GnRH stimulates the release of FSH from the pituitary, which causes a follicle cell within the ovary to mature. As the follicles develops, estrogen is secreted promoting the development of the endometrium to prepare for the egg to implant. Rising levels of estrogen then inhibit the release of FSH from the pituitary. 

Ovulation

Day 14. Estrogen also triggers a sharp increase in LH production, which signals the mature ovum to burst from the follicles. Once this happens, estrogen levels begin to decline and the follicle begins to form the corpus luteum.

Luteal Phase

Days 15-28. Characterized by the development of the corpus luteum, which is the eggless follicle. Once the corpus luteum forms it begins to secrete estrogen and progesterone. Estrogen continues to promote the development of the endometrial lining. Progesterone stimulates the development of the endometrial lining, but it also inhibits LH, which prevents further ovulation and firming of the cervix. If fertilization does not occur, the corpus luteum degenerates, estrogen and progesterone drop and oxcytocin surges causing uterine contractions and the endometrial lining is shed