Changes During the Menstrual Cycle
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| Photo credit: MerckManuals.com |
The menstrual cycle begins with menstrual bleeding (menstruation), which marks the first day of the follicular phase. Bleeding occurs when levels of estrogen and progesterone decrease, causing the thickened lining of the uterus (endometrium) to degenerate and be shed.
During the first half of this phase, the follicle-stimulating hormone level increases slightly, stimulating the development of several follicles. Each follicle contains an egg. Later, as the follicle-stimulating hormone level decreases, only one follicle continues to develop. This follicle produces estrogen.
The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels. Luteinizing hormone stimulates egg release (ovulation), which usually occurs 16 to 32 hours after the surge begins. The estrogen level peaks during the surge, and the progesterone level starts to increase.
During the luteal phase, levels of luteinizing hormone and follicle-stimulating hormone decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which produces progesterone. Later in this phase, the level of estrogen increases. Progesterone and estrogen cause the lining of the uterus to thicken more.
If the egg is not fertilized, the corpus luteum degenerates and no longer produces progesterone, the estrogen level decreases, the lining degenerates and is shed, and a new menstrual cycle begins.
Full article: MerckManuals.com
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Category: estrogen, FSH, Hormones, LH, menstrual cycles, ovulation
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