Why do Women Get Periods Every Month? Evolution has the answer!

"Why do women get periods?" - a question every woman asks herself when it's time for the monthly menstrual period. There have been several theories around this, ranging from ‘cleaning our system every month’ to ‘getting rid of the pathogens transported by sperms’ [1]. The right answer to this question, however, is actually quite different and maybe even unexpected.

In this article, we are going to touch upon the factors that led us (women) to evolve the monthly phenomenon of menstruation. Let’s begin. 

When did it all start - evolution of periods?

According to research, menstruation wasn’t one of the body's default processes (like breathing or excretion). It first developed in the anthropoid primate (the common ancestor between monkeys, apes and humans) about 40 million years ago [2].

Around this time, fetuses seem to have become more invasive - meaning that they drew on an increasing amount of maternal resources to survive. Understanding this interaction between a mother and her unborn child sheds some light on the need for evolution of menstruation [3].

What happens between a fetus and the mother during pregnancy

Pregnancy is a beautiful process but involves a complex mix of physical and mental changes - particularly in humans and other primates [4]. Let's take a look at some of these in detail. (Wondering what this has to do with periods? We're getting there!)

During pregnancy, there are three types of genes in the mother’s body - her own genes, the genes that the fetus got from the mother and those that it got from the father [5].

The fetus is an independent genetic entity that is focused on its own survival, extracting nutrients from the mother for its own development. The genes from the father may cause the fetus to produce proteins that are 'foreign' to the mother, triggering her immune system. In order to prevent an immune response on the fetus, the mother’s immune balance is modified, increasing her susceptibility to common viral, bacterial, and parasitic infections, as well as to autoimmune diseases [6, 7].

The biological demand (nutrition, change in body function, etc.) that the fetus puts on the mother can compromise her health. While regular access to wholesome nutrition and medical support to manage these changes are fairly accessible today, these would have a dramatic impact on the mother’s health in the early days of our evolution.

After fertilisation, the mother starts to develop a placenta in her uterus to form an interface between her and the fetus. The placenta is invasive - it penetrates the uterine lining - giving the fetus direct access to the mother's bloodstream to receive nutrients and oxygen [8].

In most mammals, this exchange is either pretty limited or in complete control of the mother, to the extent that they can even expel or absorb their pregnancy if needed [9]. However, this is not possible in humans and other primates. Once the embryo is planted on our uterine walls, the placenta breaches the wall and gets directly connected to the blood supply. Cutting or hampering this connection can result in hemorrhage - the reason why miscarriages are so dangerous [8].

This invasive nature of the placenta has three main consequences [11]:

1. The fetus gets direct access to its mother’s arterial blood (which carries nutrients and oxygen throughout the mother’s body), giving it ready access to nutrients that the mother may need as well.
2. The volume of blood (and, with it, the nutrients) reaching the placenta becomes largely independent of the mother’s control.
3. The placenta is not only able to take nutrients but also release hormones and other substances into the mother’s bloodstream. This enables the fetus to manipulate maternal physiology (the mother’s body function) for its benefit. For example, the exchange of resources across the placenta can influence the production of insulin in the mother's body, keeping blood sugar elevated and increasing the risk of gestational diabetes.

The innate nature of a fetus to prioritise its survival puts a lot at stake from a mother’s point of view. The ongoing need for resources continues to increase, which can cause intense fatigue, high blood pressure, and conditions like diabetes and preeclampsia in mothers [12]. Because of these risks, pregnancy is always a huge, and sometimes, dangerous investment - which brings us to our next point:

Independent of the health conditions of the parents, not all embryos formed after fertilisation are healthy. An embryo could be impaired or develop poorly because of a range of reasons, including but not limited to improper fertilisation, issues with sperm quality or the genetic makeup of the sperm or the egg [8, 13, 14, 15, 16].

A fetus will develop access to the mother's bloodstream across the placenta regardless of its health. In the event that it isn't healthy enough to survive, it can lead to a host of complications including excessive bleeding and infection, all affecting the mother's health.

Female primates evolved the phenomenon of menstruation to avoid these complications, allowing the healthiest, most viable embryos to survive while letting go of those that aren’t healthy enough. 

Why does menstruation occur and why did our body develop it in the first place?

Back to 40 million year ago. As fetuses became more invasive, mothers needed to protect their body from investing resources in an unhealthy pregnancy. They achieved this by developing a cellular mechanism inside the uterus (the uterine lining), which allowed the mother to sense an embryo's quality upon implantation.

This uterine lining (the endometrium) has cells that trigger a strong response against impaired embryos, which breaks the lining and prevents the implantation of the embryo. 

This is what happens:

After ovulation, high levels of estrogen and progesterone trigger cellular changes in the uterus, causing cells of the endometrium to change into decidual cells [17]. These decidual cells are basically biosensors of the embryo's quality, and can trigger a strong response against impaired embryos [18].

When these decidual cells 'sense' a healthy embryo, they let it attach to the uterus and begin to form a placenta. If not, the endometrium breaks - leading to periods, which prevents embryo implantation.

Through this transformation of endometrial cells into decidual cells, our species evolved an intricate mechanism to reduce the risk of unhealthy pregnancies, ensuring maternal health.

Also read about the syndrome that is PCOS.

Why do women have periods every month?

While we may understand that this process is necessary for our own safety, it doesn’t really explain why it needs to happen over and over again every month. Well, there’s no definitive answer to this, but there are a few theories.

One theory, in simple words, states that maintaining the endometrium lining indefinitely would require a lot of energy. Developing these cells only when there is a high possibility of fertilisation, during ovulation, makes the process more efficient and less of a drain on the female [19].

Unlike what it may seem, the temporary inconvenience and loss of resources due to menstruation is actually a lower cost than the expense of constantly maintaining the endometrial lining.

Another theory, which is definitely interesting, is that many species synchronise their reproductive behaviour with particular phases of the lunar cycle. Researchers hypothesise that primate and human reproductive behaviour synchronised with the moon (which has a cycle of 27 days), given that it was the only source of light at night for most of our evolutionary history [20].

While we can’t say with certainty as to the reason for this frequency, we are fairly certain that it conferred our species with a survival advantage, allowing us to survive millenia [8]. While the experience may not feel like it, in essence, the evolution of periods happened for the right reasons.

We hope this article was as interesting for you to read as it was for us to research. We definitely learned a lot more about our bodies and hope that it helped you find the answer that you’re looking for. If you still have a question that you’d like us to answer for you, feel free to reach out to us in the comments below. :)

Also learn about - PMS – Causes Of Premenstrual Syndrome And Tips For Relief!