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Humans have been getting their periods since, well, forever. So you might think that's just something that comes along with being a mammal.

But… it's not. Few mammals get periods every month, or at all. Like, that bleeding thing that female dogs do?

It's not menstruation. It comes from the vagina, not the uterus, and happens before ovulation, rather than after. As far as true menstruators go, there are only a few non-human primates, some species of bats, elephant shrews, and an African mouse.

That's it. Which just seems cosmically unfair. And it begs the question: why?

After all, you'd think bleeding once a month would be a really good way to attract predators, not to mention a waste of perfectly good reproductive tissue. Well, scientists have had several ideas over the centuries. Some of which have been less great than others.

But they seem to finally be getting somewhere when it comes to understanding what menstruation really is and why it happens. And that's not only great because it adds to our understanding of our bodies, it could also help doctors treat conditions that stem from the reproductive system. Now, part of the reason menstruation is so poorly understood is very human.

It's that people throughout history have been quite squeamish about periods. Even talking about “that time of the month,” like today, in the 21st century, is considered taboo in some circles. So we still use euphemisms around the world to describe the fact that half of all people between puberty and middle age have blood and other fluids flow from their vaginas for a few days each month.

But we here at SciShow are not too squeamish to talk about, well, most things. So we are going to talk about “that time of the month”,. Aunt Flo, the crimson tide, shark week, whatever you want to call menstruation.

Blood. Folks. There's blood.

And also some other stuff. And cramps. And weird poops.

So thanks for that, nature. In humans, and those other animals we mentioned, menstruation is a month-long cycle regulated by hormones. Technically, it starts with the bloody part.

The lining of the uterus, or the endometrium, is shed and expelled, so a mixture of blood, endometrial cells, mucus, and other tissue flows out of the vagina for three to seven days. Over this period, hormones slowly trigger the ovaries to produce around five to twenty tiny sacs called follicles, each of which contains an immature egg cell. And after a few days of maturing, just one or two of those follicles grows in diameter and becomes dominant.

It gets ready to release a mature egg cell, while the rest disintegrate. At that point, a sudden surge in luteinizing hormone prompts the dominant follicle to release its now-mature egg so it can begin its journey down the fallopian tube. So, great, you've got an egg!

Next, the body creates a nice place for it to land. In this part of the process, the spent follicle turns into a tiny hormonal powerhouse called the corpus luteum, which releases hormones that act on the lining of the uterus. They tell it to differentiate into the types of tissues needed for a fertilized embryo to implant and grow; a process called decidualization.

In non-menstruating mammals, this part of the cycle only happens when a fertilized embryo attaches to the wall of the uterus. But in humans, that process happens every month, and it starts before the egg is even fertilized. Which is why scientists call what we do spontaneous decidualization.

And now, many think that's ultimately why we menstruate. See, after all that hard work, if the egg doesn't get fertilized and implant, the corpus luteum shrinks and dissolves. And since the corpus luteum stops producing the hormones that maintain that thick, decidualized uterine lining, the uterus sheds the extra bits, kicking off another crimson tide.

Before we knew all this, of course, people had some interesting ideas about the menstruating body. And by “interesting,” we mean “pretty preposterous mythology” at the expense of, well, half of humanity. For example, scholars in ancient Rome thought that contact with menstrual blood could turn crops barren, kill hives of bees, and drive dogs crazy.

Even when people who considered themselves scientists first started studying menstruation in earnest, they did a pretty lousy job of it. Like, in 1920, a doctor coined the term menotoxin to denote the supposedly toxic substance in menstruating people's sweat. He, because, of course it was a he, observed that flowers wilted when handled by a menstruating nurse, so clearly, she was emitting some terrible stuff.

That led to the actual hypothesis from scientists that that's why people menstruated: to get rid of awful toxins that had built up inside them. Research into menotoxin continued for decades. Except … there's no such thing as menotoxin.

Scholars today think that the whole idea came about and persisted simply because of ingrained misogynistic biases. Still, this whole idea that menstruation was related to something toxic could be one reason why the first sort-of plausible, but ultimately-debunked, hypothesis was that menstruation evolved to flush out pathogens. Though, this time, they were thought to be sperm-borne pathogens.

The idea, proposed in a 1993 paper, hinged on the fact that bacteria from the genital tract are known to cling to the tails of sperm cells. And if sperms are squirmy little pathogen carriers, then menstruation could have evolved to dislodge those hitchhikers and flush them out of the uterus. But, sperm-borne pathogens are a thing in species that don't menstruate, too.

And in 1996, a scientist analyzed all of the data they could find regarding pathogens in the uterus and periods, and found that neither the number nor abundance of them decreased following menstruation. So that kind of put a nail in that coffin, and the entire notion that menstruation has anything to do with “cleaning” the uterus. Instead, that scientist proposed that monthly menstruation evolved for a much simpler reason: because it's more energy and resource-efficient.

The idea here was that we know the uterine lining has to be somewhat ready before implantation can happen. And species could keep their linings in a perpetual state of readiness, and therefore be ready to get pregnant whenever. Woo hoo!

The problem is, maintaining that lining all the time would take a lot of energy. It's actually less costly, from an energy standpoint, to grow a new, implant-ready lining each month. So, we evolved to do that instead.

That does sound logical. But it's ultimately not a great explanation for menstruation, mostly because no species of mammal we know of has a constantly-ready uterus. As far as we can tell, every non-egg-laying mammal that has ever lived has had a menstrual-like cycle where the lining builds and recedes.

Others are just way better at absorbing the extra tissue they make. Presumably because, without decidualization, they don't make as much of it. So, that brings us back to the big question:.

Why do humans have periods? Why did we go from decidualizing when an egg implants to doing it spontaneously? The current hypothesis is this: that menstruation is a byproduct of the evolutionary struggle between the fetus and its host.

And yes, the word “host” kind of makes it sound like fetuses are little parasites… because they kind of are. They're a somewhat genetically-distinct organism that's dependent on another, and whose wants and needs might not line up perfectly with those of the parent they're inside of. And depending on the species, fetuses vary in just how parasitic they are.

There are around four thousand mammals, including us, that deliver nourishment to their young before birth through a placenta, an organ attached to the wall of the uterus. All placentas are temporary. They grow at the beginning of pregnancy, and are expelled immediately following birth.

But that's pretty much where the similarities end. More to the point, there's a lot of variability when it comes to the invasiveness of a placenta, or how deeply it attaches itself to the uterine wall. A horse or pig's placenta, for example, only superficially squishes up against the uterine wall.

Dog and cat placentas go a bit deeper, but still don't make direct contact with the maternal bloodstream. But lucky us, we have what's called a hemochorial placentation. It's the most invasive kind of placenta, where the fetal tissue burrows through and actually erodes some of the maternal endometrial tissue to make direct contact with the host's bloodstream.

And scientists think that spontaneous decidualization could have evolved to mediate this relationship. There are two main hypotheses for how that would work. The first is that, since spontaneous decidualization is driven by the host's hormones instead of signals from the fetus, it could help the host keep a bit of control over their resources.

If the uterine lining is wholly responsive to the fetus, there's not much standing in the way of that little parasite becoming too attached, taking too much from the host, or destroying too much of the host's tissues. And that's especially important if you have an already-invasive placenta. The second hypothesis is that instead, or in addition to preventing the fetus from exploiting its parent, decidualization helps people deliver more healthy babies.

See, researchers know that human embryos have particularly high rates of aneuploidy, which is when there are too few or too many copies of any specific chromosome. Aneuploidies don't often keep an embryo from implanting and growing into a full-term baby, but most of them would prevent that baby from surviving. It therefore makes sense for the uterus to have some way of detecting that kind of genetic red flag early on, so it can terminate the pregnancy earlier and save resources.

At least, from a purely evolutionary perspective. Miscarriages can be so hard, no matter what biology says. But ultimately, spontaneous decidualization may help make those calls.

This is what's known as the choosy uterus hypothesis. And there does seem to be some evidence for it. Like, research suggests that shortly after they burrow into the uterine wall, embryos with lethal aneuploidies are more metabolically active than healthy ones.

Probably since they're doing their best to stay alive despite having a lethal number of chromosomes. It's thought that there's something special about the decidualized cells in the outer layers of the endometrium that allows them to sense this overactivity. And when they do, they basically slough off, leading to early miscarriage.

The remaining piece of the puzzle is understanding exactly how the endometrium being decidualized prior to implantation makes a difference when it comes to choosing embryos. And if we figure that out, it wouldn't just explain why we menstruate, it could help treat many cases of infertility and recurrent pregnancy loss. In the end though, there's more work to be done, scientists are somewhat confident that our monthly crimson tide is a side-effect of spontaneous decidualization.

And that, in turn, evolved because of the delicate evolutionary balance between being able to get and stay pregnant and not get totally ransacked by the fetus. Now, they want to understand how spontaneous decidualization actually evolved. That should help quench our curiosity about why the heck half of us get a monthly visit from Aunt Flo.

Nobody really likes her, we just tolerate her because we have to, am I right? And it should also help us better understand how the reproductive system works in general, so we can find ways to help people whose uteruses and ovaries don't work as well as they'd like or make them sick. Thanks for watching this episode of SciShow!

And thanks to Rolando, who asked about this through our Patron Quick Question Inbox. It turned out the answer wasn't so quick, but that's OK! We enjoyed answering it all the same.

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And I may be a bit biased, but I think those people are really awesome. And if you're curious, you can learn more about our patron community at Patreon.com/SciShow. [♪ OUTRO].