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The menstrual cycle is the time between the first day of a menstrual period and the last day before the next period (average 28 days). The regularity with which your periods occur, the amount of bleeding and the length of your periods provide information about whether or not the reproductive organs are working normally. This is why it is important to keep a
menstrual diary.
What happens in the reproductive organs
Month after month, several follicles begin to mature in the ovaries. Each follicle consists of the actual egg (ovum) surrounded by a group of hormone-producing cells. These cells produce the estrogens
which are required for further growth of the follicle. The estrogens also prepare the lining of the womb for implantation of a fertilized egg. Its thickness increases to about 8 mm.
Only one follicle develops to complete maturity and finally contains a mature egg. On about the 14th day of the cycle
ovulation takes place. The follicle bursts, releasing the mature egg. This is taken up by the fallopian tube which is where
fertilization takes place. The sperm can only travel up into the fallopian tubes around the time of ovulation when the thick mucus which normally seals the cervix becomes thinner and more permeable under the influence of the estrogens.
The remaining cells of the ruptured follicle which are left behind in the ovary are transformed into the corpus luteum ('yellow body') which produces the hormone progesterone. The progesterone increases the supply of blood and nutrients to the lining of the womb (endometrium) so that it is finally ready to receive a fertilized egg. If fertilization has taken place the egg can now implant in the endometrium. Progesterone then becomes the predominant
hormone of pregnancy.
If the egg is not fertilized it dies and the corpus luteum withers. The falling levels of progesterone lead to the atrophy and shedding of the uterine lining. Menstrual bleeding begins and with it the next cycle.
The role of the female sex hormones
How does the body ensure that the estrogens and progesterone are always produced at exactly the right time and in the required amounts?
Production of the sex hormones is controlled by the hypothalamus and the pituitary gland. The hypothalamus sends a releasing hormone to the pituitary gland. This is the signal for the pituitary to release two hormones: FSH (follicle stimulating hormone) and LH (luteinizing hormone). FSH and LH are carried to the ovaries via the bloodstream. In the first half of the cycle FSH leads to maturation of the follicles.
While they are maturing the follicles produce estrogens. Estrogen production increases steadily up to the middle of the cycle. The estrogens travel via the bloodstream to the reproductive organs and also to the hypothalamus and pituitary. As a result of the rising estrogen levels, release of FSH by the pituitary is increasingly suppressed, while production of LH increases. Persistently high estrogen levels in the blood cause the pituitary to release large amounts of the hormone LH at mid-cycle. This massive LH release (LH surge) triggers ovulation on about the 14th day of the cycle.
Under the influence of LH the corpus luteum in the ovary now produces progesterone - the predominant sex hormone of the second half of the cycle. Progesterone travels via the bloodstream to the genital organs and to the hypothalamus and the pituitary gland. Progesterone suppresses the hormone production by the pituitary and ensures that no further follicles mature in the ovaries. If fertilization has not taken place the corpus luteum withers and production of progesterone is drastically reduced.
The low levels of progesterone and estrogen in the blood are the signal for the pituitary to resume production of FSH and LH. A new cycle begins.
The balance of this control system of hypothalamus, pituitary and ovaries is sensitive and is influenced by numerous factors such as emotions, stress, environmental factors and disease.
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