MY DOCTOR TELLS

Now, once periods start, the ovaries will start ovulating (irregularly at first) and eventually the ovaries will produce an egg (this event is called ovulation) once every 28 days, give or take a day or two. During this developmental period of time, the vagina and uterus are lengthening and developing glands in their walls that will aid in intercourse and movement of sperm up the reproductive tract and in making it possible for the uterus to receive and sustain the new embryo when fertilization has occurred.

Given the times above this could be anywhere between the ages of 9 and 18, a rather long range in the period of time during which the female body matures, and each girl will be different in her age at the onset of the first period.

What the uterus is doing every 28-day cycle is getting ready to receive and nourish the embryo if one is available as a result of intercourse at the time of ovulation, when a sperm and egg can get together. The inner wall of the uterus, called the endometrium, grows rapidly by cell division and not only get thicker but also develops endometrial glands that produce just the right environment with all the needed nutrients for the potential new embryo if it arrives.

So when there is no embryo and the signal to maintain the steroid hormone production is thus not present, the endometrial maintenance hormone (mainly progesterone) declines to very low levels and the endometrium is destabilized. That means the blood vessels begin to break down and the extra thick endometrial wall begins to slough off. This is the menstrual flow and consists of blood, blood clots, and dead endometrial cells that leave through the uterine cervix and proceed out the vagina to be disposed in the trash. For a few days then the uterine wall is really much like an open wound, as this tissue deteriorates and drops away from the underlying wall that will remain.

What happens when the periods are missed?

The whole process will repeat itself every cycle during the reproductively competent phase of a woman’s life, generally lasting about 40 years. Once the ovaries run out of eggs, the hormones that stimulate the endometrium are no longer made by the ovary, because the cells that surround the eggs are where these steroids are made and these special cells are lost with each egg when it ovulates, and cycling with resultant periods cease. This is called menopause and a woman can, normally, no longer have babies after that time.

When an embryo is present, it will implant in (that is like digesting its way into) the endometrial wall and the placenta will start to develop so that the new life will be maintained for the 38 to 42 weeks necessary for full development of the baby. If an embryo is not present, which is most often the case, the buildup of the endometrium is not needed for that month. The hormones that caused the endometrium to develop and become functional (these are steroid hormones called estradiol and progesterone) are greatly reduced if the reproductive system lacks the necessary chemical signal (its a hormone called human chorionic gonadotropin, hCG, from the placenta) that an embryo produces when it is present and implanting in the uterine wall. [By the way, this is the chemical hormone that is tested for when a woman takes a pregnancy test. If hCG is present, an embryo is present, and the little ring or spot will turn blue in most of the tests available today.] This gonadotropin signal, hCG, is produced by the embryo in conjunction with the placenta, to stimulate the production of the steroid hormones estrogen and progesterone, and obviously can’t exist without the embryo.
INTERESTING NOTE
What happens when sex is performed during menses?
1
The chances of pregnancy are zero. It is safest period for sex for people who do not want pregnancy. The last day or two of the menses the discharge is minimal, spotting.
2
There is another thing that could happen if sex is done during menses.
An interesting side point here is that sexual intercourse during this period of time can allow the sperm, or sperm proteins, to get into the blood stream of the female and this can stimulate a strong response by her immune system for antibody production against the male’s sperm. This can result in infertility that can last for a long time, many months to many years, before the woman’s antibody “titer” goes down to low enough levels where she doesn’t kill, or at least, incapacitate the male’s sperm after any intercourse.

Try the links in the MadSci LibraryFor more information on General Biology.
Re: Why do girls have menstruation cycles?
Date: Tue Sep 21 11:54:59 1999
Posted By: R. James Swanson, Faculty, Biological Sciences, Obstetrics & Gynecology, Old Dominion University
Area of science: General Biology
ID: 936837685.Gb
MadSci Network: General Biology
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