The menstrual cycle of a fertile woman is caused by hormones. During this time, the ovaries and the uterus react to hormones such as estrogen and progesterone that are controlled by the pituitary gland and the hypothalamus. Estrogen causes the uterus lining to build up, and a drop in progesterone levels causes this lining to break down. The average length of a menstrual cycle is 28 days. It is counted from the first day of menstrual bleeding.
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The follicular phase begins from the first day of menses and ends at ovulation. During this stage, the ovarian follicles mature. The pituitary glands secret a follicle-stimulating hormone, or FSH. The amount of this hormone begins to increase during the last days of the menstrual cycle and significantly increases during the first weeks of the new cycle. FSH causes the ovarian follicles to start growing more rapidly. Around days five to seven, one of these follicles is selected to ovulate and the rest of the follicles die. The dominant ovarian follicle causes an increase in estrogen production, which thickens the lining of the uterus. Of all the stages of menstruation, the follicular phase varies the most, but typically lasts approximately 13 to 14 days.
The follicular phase ends when the level of luteinizing hormone, or LH, suddenly increases. This causes ovulation, or the release of the now mature egg cell approximately 10 to 12 hours after the LH peak. The egg cell only lives for 12 to 24 hours after ovulation if it does not get fertilized. The ruptured follicle closes after the egg cell has been released and turns into a structure called a corpus luteum.
The luteal phase last approximately 14 days and ends just before a menstrual period. In this phase the corpus luteum prepares the uterus for pregnancy by producing, for example, progesterone. This causes the endometrium to thicken. Estrogen levels are also high in this phase causing the breasts to feel tender and swollen.
The corpus luteum needs continued LH support. If pregnancy does not occur, the corpus luteum degenerates at the end of the luteal phase in a process called luteolysis. In this process, the corpus luteum forms scar tissue and becomes corpus albicans. The corpus luteum typically degenerates after 14 days if there is no pregnancy. This marks the start of a new cycle.
If the egg is fertilized and it implants, the production of human chorionic gonadotropin hormone, or HCG, begins. If this hormone is released, it keeps the corpus luteum alive. Thus, the progesterone production continues and the lining of the uterus keeps thickening. If a woman gets pregnant, the corpus luteum is called the corpus luteum graviditatis. Later in pregnancy, the placenta takes over the progesterone production and the corpus luteum degrades.