A small part of the species on Earth share a seemingly mysterious feature: a menstrual cycle. We are one of the few chosen. Monkeys, bats, humans and elephants are the only mammals on Earth to menstruate. We also do it more than any other animal, although it is a nutrient loss and can be a physical nuisance.

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

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

This evolutionary conflict of interest puts a woman and child unborn in a biological warfare that plays within the womb. One factor that contributes to this internal clutch is the placenta, the fetal organ that connects to the mother's blood supply and feeds the fetus as it grows.

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

Through the placenta, the fetus pumps the mother's arteries with hormones that keep them open to provide a permanent flow of nutrient-rich blood. A fetus with such an unrestricted approach can produce hormones to raise blood sugar in the mother's blood, to dilute the arteries, and to inflate blood pressure.

Most mammalian mothers can expel or reabsorb embryos if required, but in humans, as the fetus is linked to the blood supply, discontinuation of this connection can result in hemorrhage. If the fetus develops poorly or dies, the mother's health is compromised.

As it grows, the constant need of a fetus for resources can cause great fatigue, high blood pressure and conditions like diabetes. Because of these risks, pregnancy is always a big, and sometimes dangerous, investment. So it makes sense that the body should carefully control the embryos to find out which ones are worth challenging.

Pregnancy begins with a process called implantation, where the embryo is implanted in the endometrium that runs the uterus. The endometrium evolved to make implantation difficult, so that only healthy embryos could survive. But in doing so, she chose the most energetic embryos, creating an evolutionary reaction loop.

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

What happens when an embryo fails? It can still be attached. As it dies slowly, it can leave the mother vulnerable to infection, and all the while, it can release hormonal signals that damage her tissues. The body avoids this problem by simply removing any potential hazards.

Whenever ovulation does not result in a healthy pregnancy, the uterus removes any eggs, diseased or dead embryos. This protective process is known as menstruation. This biological feature helps to perpetuate the human race.