What is the Purpose of a Woman’s Period?

Woman’ Period: A handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We’re one of the select few. Monkeys, apes, bats, humans, and possibly elephant shrews are the only mammals that menstruate.

We also do it more than any other animal, even though it’s a waste of nutrients and can be a physical inconvenience.

Why Do Women Get Periods?

The Double-Edged Sword of Pregnancy

The answer begins with pregnancy.

During this process, the body’s resources are cleverly used to shape a suitable environment for a fetus, creating an internal haven for a mother to nurture her growing child. In this respect, pregnancy is awe-inspiring, but that’s only half the story

Also Read:  6 Reasons for Late Period Besides Pregnancy

The other half reveals that pregnancy places a mother and her child at odds.

A Genetic Tug-of-War

As with all living creatures, the human body evolved to promote the spread of its genes. For the mother, that means she should try to provide equally for all her offspring. But a mother and her fetus don’t share exactly the same genes.

The fetus inherits genes from its father as well, and those genes can promote their own survival by extracting more than their fair share of resources from the mother.

This evolutionary conflict of interests places a woman and her unborn child in a biological tug-of-war that plays out inside the womb.

The Placenta’s Power Play

One factor contributing to this internal tussle is the placenta, the fetal organ that connects to the mother’s blood supply and nourishes the fetus as it grows. In most mammals, the placenta is confined behind a barrier of maternal cells, allowing the mother to control nutrient flow.

But in humans and a few other species, the placenta penetrates directly into the mother’s circulatory system, accessing her bloodstream without restriction.

Through its placenta, the fetus pumps the mother’s arteries with hormones that keep them wide open, ensuring a constant flow of nutrient-rich blood. A fetus with such unrestricted access can manipulate the mother’s body, increasing her blood sugar, dilating her arteries, and raising her blood pressure.

A High-Stakes Investment

Most mammal mothers can expel or reabsorb embryos if needed, but in humans, once the fetus connects to the blood supply, severing that link risks severe hemorrhage.

If the fetus develops poorly or dies, the mother’s health is endangered.

As the fetus grows, its relentless demand for resources can lead to intense fatigue, high blood pressure, diabetes, and conditions like preeclampsia. Given these risks, pregnancy is always a huge and sometimes dangerous investment.

So it makes sense that the body would carefully screen embryos, accepting only those strong enough to justify the challenge.

The Endometrial Gauntlet

This is where menstruation comes in. Pregnancy begins with implantation, where the embryo embeds itself in the endometrium, the lining of the uterus.

The endometrium evolved to make implantation difficult, ensuring only the healthiest embryos survive. But, this selectivity also favored the most aggressively invasive embryos, creating an evolutionary feedback loop.

Why the Body Hits “Reset”

When an embryo fails to meet the body’s standards, it may still partially attach or weakly implant. As it slowly dies, it could leave the mother vulnerable to infection or disrupt her tissues with harmful hormonal signals.

To avoid this, the body takes a drastic approach: it sheds the entire endometrial lining whenever ovulation doesn’t result in a healthy pregnancy. This purge removes unfertilized eggs, sick embryos, and any other potential threats.

Also Read: The Fascinating Science of the Menstrual Cycle

A Bizarre but Brilliant Strategy

That protective process is menstruation, leading to the period. This biological trait, bizarre as it may seem, ensures that only the strongest embryos survive, setting the stage for the continuation of the human race.

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