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Normal menstrual cycle

Autor: doc dr Milan Terzić   


Menstrual cycle starts on the first day of a single menstrual period, and ends on the first day of the next period. The releasing of the egg cell, the only cell visible for the eye, occurs approximately midway through the normal menstrual cycle.

Before she is even born, the ovaries of a girl are full of all cells which will mature and be released during her life. Three to four months before birth, a female fetus has approximately a 7 million egg cell, and in the time of the birth only one million is left, and the others are absorbed. When a girl gets her first menstruation (menarche) the number of her egg cells is about 400.000. Later in life, during her menopause, there are just several the egg cells left. Normally, every month only one egg cell matures in the ovaries.

What happens during the menstrual cycle?

During a single cycle (month), while menstrual bleeding, there are several follicles in the ovaries that are developed. The follicles are bags with a single immature egg cell inside. In about a week, one follicle is rapidly developed and becomes larger than the others, which shrink. During this phase more and more estrogen is produced, mainly in cells that surround the dominating, growing follicle. The estrogen affects mucous membrane of the uterus (endometrium) and causes its growth and thickening. One or two days before the ovulation (discharging an ovum) the level of estrogen reaches its highest values. The concentration of estrogen is afterwards reduced and the dominant follicle produces another hormone, progesterone. The follicle swells, ruptures and releases the egg cell. The body temperature is then increased and the secrete in the glands of the cervix is changed.

The egg cell enters the fallopian tube and moves towards the uterine cavity. This lasts for 6 days. Of there are spermatozoa in the fallopian tube, the fertilization is possible.  The fertilization happens usually within 24 hours after ovulation.

During the period the egg cell travels to the uterine cavity, the follicle is being transformed. The follicle grows again and transforms itself into a "yellow body" and produces large quantities of progesterone and estrogen. Influenced by these hormones, the endometrium continues to grow. The mucous membrane gains a large number of small blood vessels which makes it very vascularized. At the end of the menstrual cycle the size of uterine mucous membrane is almost doubled, with plenty of nutritious matters that are needed for the fertilized egg cell.

How does the fertilization happen?

After the egg-sperm adhesion, the egg cell is implanted in the prepared mucous membrane of the uterus. The hormones which are produces by the yellow body support this process. If the fertilization does not happen, the egg cell is disintegrated and eliminated from the body with virginal secret, usually before the menstrual bleeding. Two weeks after the ovulation, the yellow body reduces its size and the level of estrogen and progesterone is decreased.  The uterus starts to shed the superficial layer of endometrium and discharge it by menstrual bleeding.

So, the menstrual material is not only the menstrual blood, but also the secretion from the cervix and vagina, as well as the shed layer of the endometrium. On the forth of fifth day of the menstrual bleeding the endometrium is thin and the process of its growth starts all over. The menstrual bleeding stops when the external layer of endometrium is completely restituted.

The normal duration of the cycle

The cycle of most women lasts for 28 days, although normal cycles last 21 up to 35 days. If your cycle is not within this range, it can still be "normal", although extremely long or short cycles are usually associated with infertility. The menstrual bleeding lasts 4 days, although the normal range is 2 up to 7 days.

In some period of life the menstrual cycle may be irregular, for an unknown reason. These changes are usually in the connection with the age of a woman: the cycles are longer after the menarche, and when approaching the menopause. However, sometimes stress may have significant impact. The menstrual cycle is regulated by hormones controlled by hypothalamus. Being of a complex structure and regarding the fact that mental and physical stress influences the hypothalamus, it is very clear that there are numerous factors that have effect on the menstrual cycle.

Different entities are going to be presented in the today's issue that will help you distinquish the "normal" menstrual disorders from those that need medical attention.  

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