Why do women menstruate?

We already know that

Each month, in response to reproductive hormones—primarily estrogen and progesterone—a woman’s uterus prepares for pregnancy. The inner lining of the uterus, called the endometrium, thickens, divides into different layers, and develops an extensive network of blood vessels to receive an embryo. If a woman does not become pregnant, progesterone levels begin to drop. The thick endometrial tissue and its blood vessels then begin to slough off and pass through the vagina. This bleeding is a period.

Who menstruates?

Among our closest relatives, the tailed monkeys and great apes (which live mainly in Africa and Asia) and the elephant shrew are the only animals that have periods. In the "extended family," menstruation also evolved independently in two other groups: some bats and elephants.

We don't know that

Why do women menstruate, but most other species that give birth to young do not? We are still searching

Toxin removal?

This was one of the first thoughts about menstruation. Much of the research in the early 20th century was colored by deeply rooted prejudices about menstruating women, some of which persist to this day.

For example, Bela Schick, a popular pediatrician of Hungarian origin, coined the term "menotoxin." In 1920, he conducted experiments in which menstruating and non-menstruating women handled flowers. Schick concluded that the former excreted toxic substances from their skin, which caused the flowers to wilt. Menotoxins, according to Schick, also stopped the growth of yeast and prevented dough from rising. However, they were unable to isolate these compounds or determine their chemical nature.

As you might imagine, such reports were very damaging to women, who were then treated as inferior, even disgusting. Attempts to study menotoxin were made until the 1970s!

Defense against pathogens from men?

In 1993, Margie Profet, then at the University of California Berkeley, suggested that the function of menstruation was to defend against pathogens carried to the uterus by sperm. Thus, contrary to the previous hypothesis, she considered men to be "unclean." She saw menstruation as a means of reducing the chance of sexually transmitted diseases. Profet's idea quickly fell through due to lack of evidence.

Energy saving?

In 1996, a new idea emerged. Anthropologist Beverly Strassmann of the University of Michigan recognized that maintaining a thick, blood-filled layer in the uterus costs a lot of energy. It is much more cost-effective for the body to expel that blood and rebuild the damage to the uterine wall. “The fact that some species lose blood is not an adaptation, but a side effect of the species’ anatomy and physiology,” Strassmann argued.

A consequence of evolution?

Colin Finn, then at the University of Liverpool in the U.K., suggested something similar in 1998. His idea was that menstruation was a necessary consequence of the evolution of the uterus, not a way to conserve energy, as Strassmann argued.

According to Finn, the embryos grow deeper and deeper into the mother's tissue, and the uterine wall defends itself against the embryo by thickening and creating layers. This thick "lining" is perfectly receptive to the embryo, but only for a few days. Then, if the woman is not pregnant, this lining must be removed.

Deena Emera of Yale University in New Haven noted in a 2011 paper that in most mammals, changes in the uterus are triggered by signals from the embryo. As a result, the uterine lining thickens in response to pregnancy, a capacity that evolved to protect the mother from an aggressive fetus.

In horses, cows, and pigs, the embryo simply lies on the surface of the uterine lining. In dogs and cats, the fetuses penetrate a little more. But in humans and other primates, the fetus burrows through the uterine lining to become completely integrated with the mother, literally bathing in her blood. Elizabeth Rowe of Purdue University in West Lafayette, Indiana, calls this an “evolutionary tug-of-war” between mother and fetus.

During pregnancy, the body wisely uses resources to create a friendly environment for the fetus, creating conditions for the mother to nourish the growing child. And that is the bright side of pregnancy. Unfortunately, it is also a fact that pregnancy puts the mother and child in conflict. The mother's job is to provide equal opportunities for all offspring.

The mother wants to ration how much nutrition she gives each child so that she has some left over to have more children. On the other hand, the developing child wants to draw as much energy as possible from its mother. The fetus, through the placenta, pumps hormones into the mother's arteries that dilate them, ensuring a constant supply of nutrient-rich blood. This can raise the mother's blood sugar, dilate the arteries, and increase blood pressure. Most mammals can expel or absorb embryos, but in a human fetus, which connects to the circulatory system, severing the connection risks bleeding. If the fetus develops poorly or dies, the mother's health is also at risk. And the fetus's constant need for nutrients and oxygen can result in fatigue, high blood pressure, diabetes, or preeclampsia.

Because of the risks, pregnancy is always a big, sometimes dangerous, investment. That's why the body carefully examines embryos to see which ones are worth the risk. When an embryo dies, it exposes the mother to infection and can still secrete hormones that damage her tissues. The body avoids this problem by eliminating the potential risk. Ifovulation does not result in a healthy pregnancy, the uterus sheds the lining that lines it, along with the unfertilized egg or the diseased, dying, or dead embryo. This protective process is menstruation, which leads to bleeding.

So what is menstruation?

In simple terms, it is a biological trait that allows the human race to survive. It is a very broad perspective. Maybe thinking this way will make it easier to survive this monthly toil?