The Science of Menstruation and Menopause, Explained by an OB-GYN

OBGYN and author Dr. Jen Gunter explains what’s actually happening inside the body during menstruation and menopause, why so much of what we’ve been told is wrong, and why that information gap has been weaponized against half the population.

I’m an active physician. I see patients in the office. I see a lot of communication issues. I hear a lot from patients online about difficulties interacting with their health care provider, or people who have fallen down rabbit holes with disinformation and gone down routes that may not have been helpful to them. And I realized some time ago that medicine had a big communication gap, and so I really view everything that I’m doing as trying to close that communication gap, trying to give people information about their body so they can be informed, because you can’t make an empowered, educated decision about your body if you don’t have the right information on this subject.

I want people to have the information so when they’re in the office with a medical provider, they can advocate for good care, so they know what’s harmful, what’s normal. And I also want people to have that information so when they encounter things they see online, they’ll know what’s factual and what’s not. So I’m very interested in educating the public about science and medicine because there’s huge gaps in communication. You can’t be an empowered patient without accurate information. So I view myself as someone trying to close those gaps by getting knowledge about your body out to people so they can advocate for themselves in the doctor’s office, and so when they see health information online, they can sort through what’s factual and what is likely snake oil.

My name is Dr. Jen Gunter. I’m a San Francisco Bay Area OB-GYN, author, and health communicator, and my latest book is Blood: The Science, Medicine, and Mythology of Menstruation.

The Science of Menstruation

For many years — thousands of years, maybe longer — society has viewed menstruation as something toxic or dirty or potentially dangerous to men, something that reflects a problem with the body. The ancient Greeks believed that men were perfectly in balance fluid-wise and women were not, and menstruation was evidence of that. They believed that every cell in the body was overly moist, and so if you can’t handle your fluid appropriately, you need basically an overflow valve — and that was really what they thought menstruation was. So it was a physical sign of inferiority. And that banner has long been carried by medicine, by many different societies, and also by many religions. It’s been weaponized against women as a sign of inferiority when really it’s simply a biological function.

“It was a physical sign of inferiority. And that banner has long been carried by medicine, by many different societies, and also by many religions.”

We know very little about how women cared for their menstrual cycle historically, because almost everything that has been written was written by men. People have gone through diaries from women in the 1700s and 1800s trying to understand what they might have done, and they find information about how many pairs of underwear you might need or details about other things, but there’s almost no information about how they cared for their menstruation. This is probably a sign of how much of a taboo it was to discuss the subject — related to the shaming of women and women’s bodies, and possibly also because perhaps it was assumed that everybody already knew how to take care of it. But we do definitely have an information gap about how women have handled periods historically.

There are so many potential negative outcomes when people don’t know about the menstrual cycle. We’ve often viewed these negative outcomes as a risk of unplanned pregnancy and not being able to time pregnancy, and that’s certainly one of the issues. However, it’s also important to understand that when people don’t know what’s happening to their body — when they don’t know what’s normal or what’s abnormal — they don’t know when to seek care. They don’t know if they should be asking for help or if what’s happening is totally expected. There’s also the issue of not knowing what the best care is when you are suffering. If you don’t really understand what’s going on and you don’t really understand the science, then the therapies might not make sense to you, or snake oil you encounter online might sound like a good idea. But if you understood the biology, you’d be saying, “Wait, no — that’s not something I should be paying attention to.”

Knowledge about contraception is very important, and that is one part of the menstrual cycle, but not everybody has a need for contraception, and there are much broader discussions to be had. People live their whole lives with a menstrual cycle, and they need to understand the biology.

Historically, education about the menstrual cycle has been dismal. Speaking to people in the United States, what they’re taught varies from nothing to perhaps very little. People don’t really learn practical information. If they do learn some things, it’s framed entirely in the context of pregnancy and preventing pregnancy, as opposed to being about the menstrual cycle itself and what menstruation means for a person’s body. It’s a very narrow view that doesn’t provide much useful information.

Everybody should care about menstruation because if you’re reading this, you’re only here because somebody had a menstrual cycle. The menstrual cycle is the wheel that drives humanity. It was an essential part of human evolution — getting the big brains that we have required a menstrual cycle. All the technology we use and everything humans have built comes to you, in some sense, because of the menstrual cycle. You might know someone with a menstrual cycle. You might love someone with a menstrual cycle. Knowing more about it could help you care for family members, friends, or partners when they’re having problems. For centuries, perhaps thousands of years, medicine has been medicine for men. We need to step back from that, be far more inclusive, and a good way to start is to learn more about the menstrual cycle.

Many people have used euphemisms to talk about periods over the years, because if it’s treated as a shameful topic, you can’t use the word directly. One term that comes up a lot is “the rag,” related to the use of rags as menstrual products. Others include “communists in the funhouse” and “a party at the Red Roof Inn.” “Shark Week” is another, and there’s something appealing about a graphic euphemism that essentially turns the idea of shame on its head — if you think it’s shameful for someone to say they’re bleeding or menstruating, they’ll say something far more vivid instead.

Where the Word “Menstruation” Comes From

The word menstruation comes from both Latin and Greek roots meaning “monthlies” or a monthly occurrence, reflecting the idea that the menstrual period was expected to come about once a month, like the moon. That doesn’t mean the menstrual cycle is tied to the lunar cycle in any way. It simply refers to a monthly occurrence — and it has nothing to do with the word “men,” which people often ask about, and everything to do with the word “month.”

Many people have historically believed that the menstrual cycle is every 28 days, and that appears in a lot of textbooks, but there is no strong scientific proof for it. That figure represents an idealized or perhaps average cycle at certain ages, but there can be wide variation. Depending on the classification system used, a normal cycle can range from 24 to 38 days, or from 21 to 35 days. There is also significant variation cycle to cycle within the same person, which many people don’t know. It could be entirely normal to have a 26-day cycle one month and a 32-day cycle the next — a 7-day swing in either direction is a normal built-in variability.

The Biology of Menstruation

Relatively few mammals menstruate. Humans and some great apes do, as do some bats, the spiny mouse, and the elephant shrew — a kind of motley crew of species. What researchers think is that menstruation evolved four separate times: once for humans and the great apes, and then three other times for the other animals mentioned. Every other mammal has an estrus cycle, which works differently — they don’t shed the lining of the uterus the way humans do.

A menstrual cycle means that the lining of the uterus has undergone a change called decidualization. In humans, this happens after ovulation occurs and is triggered by the release of progesterone. Decidualization is a real physical change in the tissue — the decidua functions something like a catcher’s mitt in the uterus, all prepared and ready for an embryo. If there’s no decidualization, implantation can’t occur.

The decidua is especially thick in humans because we have an incredibly invasive placenta — one that needs to extract a great deal of oxygen to feed those large brains. An invasive placenta has to be carefully managed, or it could grow through the uterus and cause serious problems. Humans have therefore evolved a particularly thick decidua to handle it. The decidua also plays a role in sensing embryo fitness, helping to regulate which pregnancies take hold and which end in very early miscarriage.

When a pregnancy doesn’t occur in a given cycle, that thick tissue has nowhere to go. It can’t be reabsorbed by the body, so the only way to shed it is to expel it. Menstruation involves chemical changes that cleave the decidua from the uterus, opening blood vessels that push the fluid and tissue out — and then the cycle can begin again.

In an estrus cycle, decidualization is still needed for pregnancy to occur, but the trigger comes from the embryo itself rather than happening in advance. The body doesn’t need to maintain that waiting catcher’s mitt. Once the embryo makes contact, the process begins — which means animals with estrus cycles don’t need to shed a prepared lining each month the way humans do. This fundamental difference in reproductive biology is what makes menstruation a uniquely human — and relatively rare — phenomenon across the animal kingdom.