Menstrual Cycle - How Menstruation Occurs

The menstrual cycle is a physiological process that occurs cyclically in all fertile women. The first menstruation of a woman's life is called menarche, and the last menstruation.

This text is more conducive to school work and for students in biological areas. The other text is aimed at people looking for information about the fertile period. I suggest, in any case, the reading of both, as they are complementary.

Menstrual cycle
Menstrual cycle

Menstrual cycle

The menstrual cycle begins on the first menstruation, lasts on average 28 days and ends on the first day of the next menstruation. Some women have shorter periods of up to 21 days while others have longer periods of up to 35 days. Teenage women with recent menarche may have cycles of up to 45 days, since their reproductive system is still maturing. The same prolongation can occur with women approaching menopause, when there are already signs of ovarian failure.

The female reproductive organs consist of 2 ovaries, 2 fallopian tubes, a uterus and the vagina.

The menstrual cycle is divided equally into 2 phases: follicular phase and luteal phase. To facilitate understanding, we will consider the normal cycle with 28 days, with 14 days of follicular phase and 14 days of luteal phase.

This process is complex and involves the control of several hormones, but I will try to explain it very simply so that everyone can understand the basic mechanisms of the menstrual cycle

1) Follicular phase

The follicular phase begins on the first day of menstruation, ie on the first day of the cycle. At the beginning of this phase, the estrogen and progesterone hormones are low and the uterus is menstruating, showing a thin wall (endometrium). The ovary at this stage is at rest.

At this time, the pituitary gland (pituitary gland), which is located in the central nervous system, begins to increase the production of a hormone called follicle stimulating hormone (FSH), which as its name says, stimulates the ovary follicles.

I open a quick parenthesis here. Unlike men who produce their sperm continuously, women are born with a number of eggs. There are about 450,000 and they are stocked in the ovaries, in immature form, inside the ovarian follicles.

In the presence of FSH, the follicles begin to develop, growing and maturing. Seven days after the start of the cycle, several follicles measuring between 9 and 10 millimeters can be detected on ovary ultrasound.

These follicles, now, begin to produce estrogen. As estrogen levels increase, one of the follicles becomes dominant, developing faster than others, which actually stop growing and begin to involute. The dominant follicle grows about 2 millimeters per day to the final size of 20 to 26 mm, when it completes its development. This dominant follicle is who will release the ovum at the time of ovulation.

In addition to the development of the dominant follicle, estrogen also acts on the uterus, preparing it for an eventual pregnancy. The membrane of the wall of the uterus, called the endometrium, begins to proliferate, acquiring layers, thus becoming thicker.

2) Luteal phase

The estrogen peak occurs 1 day before ovulation. At the time of maximum concentration of estrogen, another hormone from the pituitary gland is released, the luteinizing hormone (LH). We are now right in the middle of the cycle, 14th day in cases of 28-day menstrual cycles.

The estrogen peak and appearance of LH, causes the woman to begin to produce a viscous mucus, called fertile mucus, that favors the mobility of spermatozoa.

The release of LH completes the maturation process of the dominant follicle, and approximately 36 hours after its release, follicle disruption occurs and the release of the ovule, that is, the ovule woman. Click on the image to enlarge the table of the menstrual cycle.

Some women experience pain at the time of ovulation due to irritation of the peritoneum after ovarian follicle rupture. This pain is located in the lower abdomen, on the side of the ovary that ovulated. This pain can last for a few hours and is usually cyclical, occurring almost every month. This syndrome is called mittelschmerz, which means middle (cycle) pain in German.

Soon after the LH peak that induces ovulation, the body temperature of the women rises discreetly, about 0.5ÂșC, remaining thus for another 10 days.

Temperature medication is a good method of retrospectively knowing if the woman has recently ovulated, but this rise is often late in relation to ovulation, not serving to indicate the right time for intercourse.

Corpus luteum

Just before ovulation, the cells around the ovarian follicle, under the influence of LH, begin to form the corpus luteum, structure responsible for the production of estrogen and, mainly, progesterone. When the woman ovulates, the egg is released towards the tubes, leaving only the corpus luteum in the ovary.

Throughout the final part of the luteal phase, the corpus luteum remains producing progesterone, which acts to inhibit the secretion of LH by the pituitary gland. In the uterus, progesterone acts to stop the proliferation of the endometrium. In fact, progesterone organizes the layers of the mucosa making it more homogeneous, leaving it rich in fluids and nutrients for the potential fetus. When the uterus is ultrasound in the proliferative phase of the cycle, it is possible to detect 3 different layers of the endometrium. When the same examination is done in the luteal phase, there is no such distinction and the endometrium is all uniform, ready to receive the fertilized egg.

Because progesterone inhibits LH, if the ovum is not fertilized, the fall of LH causes the corpus luteum to begin to involute, which in turn causes the production of progesterone and estrogen to decline. Without these hormones the thick wall of the endometrium no longer holds, its blood supply is cut off, and it eventually collapses, characterizing menstruation and restarting the cycle in the follicular phase.

Ovulation occurs 14 days before the first day of the next menses. So if you menstruated, count 14 days behind and you will know when you ovulated.

If the egg is fertilized, the embryo begins to produce a hormone called chorionic gonadotropin, responsible for keeping the corpus luteum and progesterone production active.

Pregnancy tests are based on the dosage of gonadotropin concentrations.

Fertile period

Upon its release, ovule only remains viable for about 12-24 hours. This means that fertilization is most likely when there are already sperm present before ovulation. Despite the short period of viability of the ovum, the fertile interval is much longer because, depending on the quality of the male semen, the spermatozoa can become viable within the female reproductive system for up to 5-7 days. Therefore, the fertile period goes from the 5th day before ovulation to a day after ovulation.

Men with lower-quality semen may have spermatozoa that survive less, thereby reducing the period of the fertile window.

The period of greatest chance of fertilization occurs when there is intercourse 1 or 2 days before ovulation.

Current work shows that semen quality is greatest when there is a 2 to 3 day interval between ejaculations. Therefore, it is indicated the coitus day yes, day does not, or every 2 days. For couples who want to get pregnant, a tip is to have sex 3 times a week, starting them right after the end of menstruation.

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