• Low blood levels of estrogen & progesterone lead to endometrial vasospasm, ischemia, necrosis and separation from basal layers; menstrual flow begins
• Low estrogen allows the hypothalamus to resume GnRH pulsation
• GnRH stimulates anterior pituitary cells to produce FSH & LH
• FSH prepares several follicles
• LH causes ovarian cells to make androgens
• Follicular cells produce estrogens from androgens
• Estrogens stimulate proliferation of endometrial cells

• FSH & LH mature one follicle
• Estrogen sustains endometrial cell and vascular growth
• Estrogen inhibits GnRH release
• GnRH inhibition temporarily halts FSH release
• Rising estrogen initiates a midcycle LH surge and
an increase in FSH
• The surge of LH causes ovulation 9 hours later (on 14th day)
• Follicular progesterone production begins

• Ovulation occurs; ovum is propelled toward uterus
• Follicle fills with blood
• Follicular cells multiply, forming corpus luteum (yellow body)
• Corpus luteum secretes progesterone and estrogen
• Progesterone & estrogen prepare endometrium for implantation
• Endometrium becomes more vascular and secretory

• Luteal progesterone continues to develop endometrial mucus glands and vasculature
• If fertilization and implantation occur, the trophoblast secretes hCG (a luteotrope)
• If fertilization does not occur, the corpus luteum involutes and progesterone and estrogen levels fall.
• Reduced estrogen & progesterone cause the hypothalamus to secrete GnRH again
•FSH is stimulated
• Minimal estrogen and progesterone cannot support the endometrium; vasospasm and necrosis ensues; menstrual flow begins