With or without pain, that's why women have period

A small part of earth's species share a seemingly mysterious feature - a menstrual cycle. We're one of several chosen ones. Monkeys, bats, humans, and elephants are the only mammals on Earth that manage. We also do it more than any other animal, even though it's a waste of nutrients and it can be a physical concern.

So where is the sense in this unusual biological process? The answer begins with pregnancy. During this process, body resources are cleverly used to form a suitable environment for a fetus, creating an inner shelter for a mother to feed her growing child.

In this regard, pregnancy is very frightening, but that's only half the story. The other half reveals that pregnancy places mother and child in disagreement. As for all living creatures, the human body evolved to promote the spread of its genes. For her mother, this means that she should strive to provide equally for all her offspring. But a mother and her fetus do not share exactly the same genes. The fetus inherits genes even from father and genes can promote their survival by extracting more than the mother's share of resources.

This evolutionary conflict of interests puts a wife and unborn child in a biological struggle that plays inside the womb. One factor contributing to this inner flock is the placenta, the fetus organ related to its mother's blood supply, and it feeds the fetus as it grows older.

In most mammals, the placenta is confined to a barrier in mother's cells. This barrier allows the mother to control the supply of nutrients to the fetus. But to humans and some other species, the placenta actually penetrates the mother's circulation system directly into her bloodstream.

Throughout the placenta, the fetus pumps mother arteries with hormones that keep them open to provide a permanent flow of nutrients - rich blood. A fetus with such an unlimited approach can produce hormones to increase sugar in mother's blood, dilate arteries, and fan tension.

Most mammals mothers may expel or reabsorb embryos if required, but in humans, because the fetus is related to the blood supply, cutting it can result in bleeding. If the fetus develops weak or dies, the mother's health is threatened.

As it grows, a fetus' constant need for resources can cause great fatigue, high blood pressure, and conditions like diabetes. Because of these dangers, pregnancy is always a big investment, and sometimes dangerous. So it makes sense that the body must carefully examine embryos to find out what is worth the challenge.

The pregnancy begins with a process called implantation, where the embryo is embedded in the endometrium that drives the uterus. Endometrium evolved to make it difficult to implant so that only healthy embryos could survive. But by doing so, she chose the most energetic embryos, creating an evolutionary reaction curve.

The embryo is involved in a complex, detailed hormonal dialogue that transforms the endometry to allow implantation.

What happens when an embryo fails? He can still join. As she dies slowly, this may leave her mother vulnerable to infection, and all the time, she can emit hormonal signals that break her tissues. The body avoids this problem by simply removing any potential danger.

Whenever ovulation does not result in a healthy pregnancy, the womb removes each egg, sick or dead embryos. This protective process is known as menstruation. This biological trait helps to continue the human race.