Why do Women Have Periods?

Author: Sparhawk Mulder Interview with Dr. Suzanne Sadedin, Ph.D.

Anyone who’s given a little thought to the question “why do women have periods” has probably realized that the classic middle-school-health-class-level explanation isn’t accurate. If “the uterus lines itself with lots of cushioning to make a nice place for the embryo,” why does it get rid of it every month? And more notably, why do only humans, some other Great Apes, and a few species of bat menstruate, while every other mammal doesn’t (or lays eggs, I suppose). And, even disregarding the problems with human birth due to our narrow hips and large heads, why are our pregnancies so much rougher and full of strange side effects such as nausea, cramps, exhaustion, incontinence, and countless other more dangerous complications? The answer lies in a fundamental misconception about Homo sapiens’ mother-child relationship.

From an evolutionary standpoint, the mother and the child have two very different priorities when it comes to distribution of resources. The mother “wants” to distribute resources roughly equally to each of her children, including future ones, and to give none to any which will die. The fetus “wants” to survive at all costs, and to take as many resources as possible. The mother-child relationship is, evolutionarily, not all lovey-dovey and harmonious; there is a dangerous conflict of interest. In most mammals, a kind of balance is struck between parent and offspring in the womb, involving the placenta, but somehow some Great Apes (and independently, some bats) evolved a way for the placenta to get ahead.

Humans have a unique kind of placenta, called a hemochorial placenta (some other apes have it too, but ours is by far the most aggressive). Most mammals’ placentas only interface with an area of thin blood vessels partially sealed off from the rest of the mother, called the endometrium. Marsupials connect even less than that. Homo sapiens’ placenta, on the other hand, burrows through the endometrium, rips into nearby arteries and redirects them directly to the embryo. Moreover, it pumps the mother full of hormones to expand the arteries’ blood flow, increasing her blood sugar levels or blood pressure. The fetus even paralyzes the nearby arterial walls so they can’t constrict (meaning that if the placenta disconnects badly, the mother might bleed out incredibly quickly during childbirth)! Instead of the mother having complete control over nutrients, her hormones, and even the fetus’ life, human pregnancies are a tug-of-war between the mother and fetus, with her endocrine system as the battlefield.

Luckily, there is also an obvious evolutionary incentive for mothers to survive as many pregnancies as possible. Moreover, the mother “wants” only the strongest embryos to survive and begin a pregnancy. Therefore, the endometrium’s lining became thicker, making it more difficult for the placenta to access the mother’s bloodstream. Consequently, embryos were selected to become more aggressive, which selected thicker uterine linings, and an evolutionary death spiral began. This means that, contrary to popular teaching…

Now the complicated part: how does a thick uterine lining (superficial endometrium) result in menstruation; why do we get rid of it every month or so? Well there are many problems with a thick uterine lining, there’s a reason it didn’t evolve in most other mammals. This thick layer must have restricted blood-flow (otherwise the hemochorial placenta could just attach there). But the longer the layer goes with low blood flow, the less receptive the tissue becomes to the mother’s hormonal signals, and thus becomes more receptive to the fetus’ signals (which would “convince” it to become more vascular and friendly to the fetus, defeating the point of the barrier layer). To make things even more difficult, that thick uterine lining is vulnerable to infection, due to the high amount of dead tissue. How did evolution fix this problem? By simply sloughing off all the extra tissue, dead or partially implanted embryos and all, whenever a healthy pregnancy doesn’t occur. After ovulation and the construction of the new uterine lining, the body waits just long enough to tell whether a healthy (albeit uncomfortable) pregnancy is begun. If it hasn’t, out goes the old lining, which is beginning to become prone to infection and hormonal manipulation. This cycle, given the development speed of Homo sapien embryos, takes about a month.

Astute biologists might wonder “but why does it happen every month?” Well, it actually doesn’t (or at least, didn’t). Hunter gatherer women, including in some groups still around today, would only experience a few tens of periods in their lifetimes, before menopause. A fair number of those would be missed due pregnancy or lactation (remember, many cultures breastfeed to much later ages, usually until four years old), but many would also be due to food insecurity or stress. None of those are good times to start feeding another mouth, and so the menstrual cycle stops. Due to unprecedented levels of nutrition, girls experience periods earlier and more regularly than ever before.

So that’s why humans menstruate, and why it’s approximately monthly.

Thank you to Dr. Suzanne Sadedin, Ph.D. for answering questions!