- Follicular (before the release of the egg
- Ovulatory (egg release)
- Luteal (after egg release)
When the follicular phase begins, levels of estrogen and progesterone are low. As a result, the top layers of the thickened lining of the uterus (endometrium) break down and are shed, and menstrual bleeding occurs.
About this time, the follicle-stimulating hormone level increases slightly, stimulating the development of several follicles in the ovaries. Each follicle contains an egg. Later in this phase, as the follicle-stimulating hormone level decreases, only one follicle continues to develop. This follicle produces estrogen.
This 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.
luteinizing hormone and follicle-stimulating hormone levels decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which produces progesterone. Progesterone and estrogen cause the lining of the uterus to thicken more, to prepare for possible fertilization. If the egg is not fertilized, the corpus luteum degenerates and no longer produces progesterone, the estrogen level decreases, the top layers of the lining break down and are shed, and menstrual bleeding occurs (the start of a new menstrual cycle).
It varies from individual to individual from 21 days to 45 days.
- Abnormal uterine bleeding
- Absence of menstrual periods (amenorrhea)
- Menstrual cramps (dysmenorrhea)
- Premenstrual syndrome (PMS)
