Menstruation is usually thought of as the periodic discharge through the vagina of a bloody secretion containing tissue debris from the shedding of the endometrium from the nonpregnant uterus.

However, as the average frequency of human intercourse, every three days or so, is high enough to make it likely that an ovulation will result in a pregnancy, this continuous sexual activity could also mean that bacteria and viruses can hitch regular free rides deep into the woman's reproductive tract.

One defense against such infection is the plug of mucus at the cervix that blocks sperm from ascending except during two or three fertile days a month. Another natural defense mechanism is menstruation. The hot flow of blood conbined with the change in pH is disastrous for pathogens and it sweeps them away from the sweet environment of the woman's innards.

In the natural environment, most women would experience far fewer menstrual cycles, since they would not cycle while pregnant or lactating, which would be most of time, given the standard of cyclical estrous and birth rates among mammals. Since humans do have sex more frequently, but have fewer offspring, menstruation is an important evolutionary defense mechanism designed to keep the females of the human race healthy.

Yes, as Zanth's writeup above details, menstruation serves a useful purpose. There's the whole business of it providing a periodic flushing of one's reproductive system to clean the germs out. And the pH decrease of menstruation is often enough to knock out the mild yeast infections that many women get.

Some biologists believe that menstruation in humans also evolved as a sign to men about a woman's impending fertility.

When you compare human menstruation with that of other mammals, ours is just plain overkill. Thus, some scientists have come to believe that all that blood is there so that a nearby male will be able to tell that the woman he's interested in is not pregnant with another man's child and will thus soon be ready to get pregnant with his child.

Menstruation is even more useful to women themselves as a pregnancy indicator.

Unmarried girls across the world feel the same flood of relief when they see the blood after a fling with a boy: Oh, thank God, I'm not pregnant!

Older women yearning for a family feel loss and sadness when they see that same blood: I'm not pregnant. Again. Damn.

So, yes, "the curse" is useful.

And it's not just a bloody mess, oh no. There's so much more to hate about it besides the blood:*

  1. PMS

  2. Chafing from thick, diaperlike pads

  3. Having to pee constantly because your innards are swollen

  4. The headaches and nausea

  5. Catching whatever crud's going around because your immune system is down for the count

  6. The need to eat Midol or ibuprofen like candy

  7. Crushing sleepiness from insta-anemia.

*ObDisclaimer: not every woman feels as if her womb has become a gateway to Hades, nor do they develop titanic cramps or other ill effects of plutonian proportion. Some women are blessed with light flows and painless periods. Drat those lucky, lucky ladies upon whom Mother Nature smiles so benevolently.

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