In 2007 Susan Brown encountered the repelling power of period blood. While studying what menstrual fluid might reveal about a woman’s health, she wanted data from a cross section of subjects beyond the student volunteers at the University of Hawaii at Hilo, where she worked as an evolutionary psychologist. Brown’s team members set up a booth near the entrance of a Walmart in downtown Hilo and hung a sign that said, “Menstrual Cycle Research.” Then they waited. All afternoon women and men would spot the sign, then gingerly skirt past without making eye contact.

About six months later Brown and her Hilo colleague Lynn Morrison presented their findings at the annual meeting of the American Association of Physical Anthropologists. A wave of “nervous twittering” broke out when Morrison described carrying menstrual blood samples down the hallway of their laboratory to analyze hormone levels and other biomarkers. “The audience was fine discussing a woman’s cycle in the abstract,” Brown explains, “but not menstrual blood itself.”

That aversion has influenced women’s relationships to their own bodies as well as how the medical establishment manages women when things go wrong with their reproductive health. “Our menstrual taboo is at the core of how this science is getting done,” Brown says of research on menstruation.

Or not getting done, as the case may be. It is hard to measure how much money is spent on period research, but experts agree the subject is underfunded. “It’s a chicken-and-egg situation, where there’s not much funding for research, so there’s also not much quantifying of that lack of research,” says Elizabeth Yuko, a bioethicist at Fordham University.

Yet period disorders are incredibly common. When Saudi Arabian researchers surveyed 738 female college students in a 2018 study, they found that 91 percent reported at least one menstrual problem: some got their periods irregularly or not at all; others reported excessive levels of bleeding and pain. Different studies show that as many as one in five women experiences menstrual cramps severe enough to limit her daily life. About one in 16 worldwide suffers from endometriosis, a disease where menstrual blood and tissue migrate outside a woman’s uterus and form painful lesions in her pelvic cavity. And one in 10 women has polycystic ovarian syndrome, a hormonal imbalance that disrupts a woman’s cycle and is a leading cause of infertility. “You can argue we need to put our resources toward researching the life-and-death stuff,” Yuko says. “But that argument falls apart because we’ve had no problem funding erectile dysfunction research.”

Menstruation, of course, is essential to human reproduction and therefore survival. It is also one of the biological processes that makes us special because humans, chimpanzees, bats and elephant shrews are among the only animals on earth that go through it. The vast majority of mammals signal fertility through estrus, the period when females are ovulating and display their sexual receptivity via genital swelling, behavioral changes or pronounced alterations in body odor. The female human body, however, conceals this critical window. Instead our most visible sign of potential fertility is menstrual blood, which, ironically, appears after the fertile period has closed. The endometrial lining of the uterus thickens over the course of a woman’s cycle as her estrogen level rises. If none of the eggs she releases at ovulation joins with a sperm and implants in that lining as a fertilized zygote, then levels of estrogen and another hormone called progesterone drop, triggering the uterus to shed the thickened endometrium so it can start fresh in the next cycle.

But beyond this basic picture, scientists are still struggling to understand fairly fundamental questions: Why do we share this process with at least six species of bats, for example, but not monkeys? And just what is menstrual blood, exactly? “It’s quite different from regular blood,” Brown notes. “We know it can’t clot and is full of immune agents, but we don’t know much about what they do.” It is also unclear why we shed this biological tissue so dramatically when most mammals that experience estrus appear to reabsorb their endometrial linings at the end of each cycle. Even less is known about why so many women—up to 80 percent by some estimates—experience cramps, bloating, fatigue, anger or other symptoms just before the onset of menstruation. “We know so little about menstruation,” says Tomi-Ann Roberts, president of the Society for Menstrual Cycle Research and a professor of psychology at Colorado College, and what scientists do know is often badly communicated with the public. “Because of this, our attitudes toward menstruation are overwhelmingly negative. This has real consequences for how we can begin to understand healthy menstruation, as well as menstruation-related disorders and the treatment options available.”

Masking menstruation

The taboo has taken many forms. In 1920 a Hungarian-born pediatrician working in Vienna named Béla Schick published a collection of anecdotal observations: When he asked a menstruating woman to handle flowers, they wilted within minutes. When he compared the bread dough made by several women, the loaf made by the one having her period rose 22 percent less. Schick concluded that menstrual blood contained a kind of poison. By the early 1950s Harvard University scientists were referring to “menotoxins” and injecting menstrual blood into animals to observe the effects. Some of those animals died, most likely because the blood samples carried bacteria and other contaminants. Not much came of these experiments in terms of useful data, but the notion that menstrual blood contains mysterious and even dangerous properties has persisted in the scientific literature and our cultural imagination.

Menstrual cups can be a reusable, environmentally friendly alternative to tampons and pads. Credit: Jamie Chung

By the late 1950s research around menstruation had shifted to center almost entirely on preventing unplanned pregnancies at a time when maternal and infant mortality was troublingly high, especially in poor communities. In 1923 Margaret Sanger, the activist, nurse and founder of the organizations that would later become Planned Parenthood, wrote that “Birth Control means liberation for women and for men.” In 1951 she met a physiologist named Gregory Pincus, who had performed what was considered at the time to be the first in vitro fertilization of rabbits. With Sanger securing funding, Pincus set up a lab to test formulations of synthetic versions of hormones that regulate the menstrual cycle and teamed up with John Rock, a Boston obstetrician-gynecologist, to run clinical trials of the drug.

After a study of almost 60 women in and around Boston, Pincus and Rock turned to Puerto Rico to run the first large-scale trial of the drug that the U.S. Food and Drug Administration would approve in 1960 as the first oral contraceptive. They recruited 265 Puerto Rican women, many of them poor, to the study without the level of “informed consent” required today. Twenty-two percent of the participants dropped out after reporting side effects such as nausea, dizziness, headaches and vomiting. The study’s medical director argued that the pill “caused too many side reactions to be generally acceptable.” Nevertheless, it went to market.

The pill was, of course, celebrated as a huge breakthrough. “It was the first form of birth control separate from sex that women could completely control,” notes Elizabeth Kissling, a professor of women and gender studies at Eastern Washington University. It is impossible to overstate the freedom the pill represented for women, whose reproductive lives were otherwise largely under male control. But liberation came with a price. By the late 1960s patients across the U.S. were reporting the same symptoms documented during the Puerto Rican trial. Despite many reformulations over the ensuing decades, side effects remain a problem for many women on the pill; risks for breast cancer, blood clots and stroke may also be higher. In their quest to bring reproductive freedom to women, Sanger, Pincus and Rock appear to have ignored the implications of shutting down a woman’s natural cycle, Kissling explains. In other words, scientists figured out how to supplant periods long before they began trying to understand why they work the way they do.

It was not until the late 1980s that scientists really began to grapple with the larger question of why menstruation happens at all. As an undergraduate, evolutionary biologist Beverly I. Strassmann wrote a paper on how concealing ovulation could entice more paternal partners. (Because a woman’s fertile window is more or less invisible, it encourages what researchers call pair-bonding: human males invest in fewer sexual relationships and protect and care for the resulting offspring as a way to ensure their paternity.) Strassmann, now a professor of anthropology at the University of Michigan, wanted to explore human attitudes toward menstruation by collecting data in a community where women spend five nights of their period sleeping in huts that are separate from the rest of the tribe.

In 1986 Strassmann moved to Mali to conduct field research on the Dogon, an ethnic group of millet farmers that hew to their traditions. Dogon people who continue to practice their indigenous religion believe that a menstruating woman’s presence would desecrate the religious objects in the family compounds. Researchers had not previously considered that these religious beliefs were rooted in any kind of reproductive agenda. But, as Strassmann explains, she hypothesized that this was “a cultural pattern embedded in religion that did directly serve reproduction.” Although research on modern indigenous communities can offer only clues about how humans lived thousands of years ago, Strassmann hoped to show that long-standing cultural taboos around menstruation had developed to support our larger evolutionary goals.

During her initial fieldwork, Strassmann studied the community’s use of menstrual huts for almost three years, collecting urine samples from 93 women to test hormone levels and prove that their use of the huts correlated with actual menstruation patterns. She also observed how quickly most of the women got pregnant again after their visits to the huts. Although the practice was ostensibly about keeping menstruation sequestered, the huts themselves were located in full view of a shade shelter used by men in the community. So the huts made a woman’s fertility status clear to her husband and his family whether she liked it or not. (As noted earlier, women enter their “fertile window” after their period.)

Other religious practices around menstruation, such as the Orthodox Jewish purification ritual of sending menstruating women to mikvah baths, can also be traced to men’s need to track female fertility and schedule sexual activity accordingly. And although the advent of the pill means that many women can now control their reproductive life in ways that render the purpose of such practices moot, the taboos still persist, Roberts says. “We still think of menstruation as something that women have to keep hidden and separate.”

Period evolution

Although Strassmann’s work was primarily about understanding the biological underpinnings of menstrual taboos, her data also revealed important characteristics about the process of menstruation itself. Perhaps her most oft-cited finding was published in 1997 in Current Anthropology: across human history, menstruation has been a rather infrequent event. That is because women tend to get pregnant earlier, have more babies and spend more time breastfeeding in communities where birth control is unavailable or difficult to access than they do in communities with high rates of birth-control usage. “We think of periods as happening 12 times a year, but if you’re pregnant and then nursing for extended time frames, that’s a stretch of two or three years for each child when you’re not menstruating,” Strassmann explains. Her data showed that in the 1980s the average Dogon woman menstruated only around 100 times in her life, compared with the average American woman’s experience of as many as 400 periods in her lifetime. And Dogon women’s experience is closer to what all women would have experienced throughout history before the development of the pill.

This historical infrequency of menstruation helps to explain why humans evolved to do something as potentially disadvantageous as releasing blood—losing iron, protein and other nutrients and probably attracting predators in the process. It could also help explain why periods and the week before their onset can be so unpleasant for many women. Michael Gillings, a professor of molecular evolution at Macquarie University in Australia, became interested in women’s experiences of premenstrual symptoms (PMS) when premenstrual dysphoric disorder (PMDD) was added to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders in 2013.

PMDD is defined as severe irritability, depression or anxiety in the week or two prior to menstruation, with symptoms easing two or three days after menstruation begins. But Gillings, along with many feminist scholars, balked at the characterization of mood swings as disordered. “Up to 80 percent of women report these symptoms; that makes PMS normal, not a psychological disorder,” he says. “So we have to ask, Was there, at some point in history, an advantage to having these symptoms?” In 2014 he published a paper in the journal Evolutionary Applications arguing that PMS offered a selective advantage because it caused tension between pair-bonds and therefore might help women dissolve relationships with infertile men. “It is difficult to prove a hypothesis like this,” he acknowledges. And the media response characterized him as insensitive to the suffering of women. “I was burned in effigy on five continents,” he says. Some researchers counter Gillings’s claim that PMS is a product of evolution—and contend that its roots are more cultural than biological because it manifests differently around the world. Roberts sees the concept mostly as one influenced by the menstrual taboo and a way to dismiss women’s emotions.

Scientists are also divided over whether the act of bleeding itself serves an evolutionary purpose. “It’s never made sense to me that we have this free-flowing blood, while other animals reabsorb it,” Brown argues. Many evolutionary biologists now think that the essential feature of women’s cycles is not the bleeding but rather the ability of the uterus to thicken its lining in preparation for implantation and then dispose of the endometrium when it is not needed. “A healthy endometrium requires constant metabolic support, so it is less energy-intensive for the female body to tear down and rebuild it each cycle than it is to maintain it in a constant state of readiness for embryo implantation,” Strassmann explains. Human circulation happens to result in a particularly bloody endometrium. “Our physiology doesn’t permit reabsorption, so much of the blood gets discharged as menstruation,” she says. Bleeding may therefore be an insignificant by-product of evolution rather than an advantage.

A world without periods?

If the act of shedding menstrual blood poses no clear health benefit or evolutionary advantage and if, historically, women have not even done it all that often, then why, in this postpill era, do women continue to do it all? The answer: some do not. In January the Royal College of Obstetricians and Gynecologists in London released new guidelines that approved skipping the placebo pills in birth control to reduce the frequency of periods or avoid them altogether.

Although this formal acknowledgment is new, the practice is not. Medical menstrual suppression has long been embraced by clinicians, the media and women frustrated by the pain, mood swings or inconvenience of their menstrual cycle. The pharmaceutical industry also took notice: as the researcher who first measured and quantified the frequency of human menstruation, Strassmann has been asked to present her data to drug manufacturers, who have offered several versions of the pill and other forms of contraception that are formulated to let women skip their periods more often, if not avoid them altogether.

Phone apps, such as Clue (shown), help women to keep track of their monthly cycles. Credit: Jamie Chung

Skipping that monthly ordeal can mean avoiding debilitating pain, prolonged heavy bleeding, migraines and other symptoms that can dramatically impair a woman’s quality of life. The approximately 25 percent of reproductive-age women and girls who struggle with additional kinds of severe menstrual pain may be at increased risk for developing other chronic pain conditions. “We suspect the cyclical experience of monthly menstrual pain somehow alters how some women process all kinds of pain,” explains Laura Payne, who directs pain research for a pediatric program at the David Geffen School of Medicine at the University of California, Los Angeles.

To many doctors faced with patients whose periods cause problems, “the pill is the closest thing we have to a panacea in women’s health,” says Jonathan Schaffir, an associate professor of obstetrics and gynecology at the Ohio State University. But is it? “The pill isn’t a treatment for these conditions,” Kissling says. “It’s a way of refusing to treat them.” It can take up to a decade or longer from disease onset for a woman to be diagnosed with endometriosis, for example, in part because doctors are so quick to prescribe the drug to teenagers reporting bad cramps without investigating to see if there is an underlying cause, says endocrinologist Jerilynn Prior of the University of British Columbia. And where one version of the pill may succeed in masking a woman’s symptoms, another may exacerbate them. “You can spend years jumping from one pill to another, not finding relief,” notes Kissling, who published a paper on how women end up “treating each other,” for better or worse, in online forums, where they share alternative medicine remedies and other tips out of frustration with their doctors’ limited repertoire.

Strassmann and many others are skeptical about the health effects of medically induced menstrual suppression, which may expose women to hormone levels higher than what they would have experienced in the evolutionary past or even now, when regularly cycling on the pill. “It’s true a monthly menstrual period is not necessary,” she says. “But taking more progestin to skip your period is not living like our ancestors did 500 or 1,000 years ago.” Research shows taking the pill reduces the risk for endometrial and ovarian cancers but slightly raises the risk for breast cancer, stroke and blood clots.

In 2017 Strassmann and her colleagues published a paper in Evolution, Medicine, & Public Health tracking how exposure to synthetic hormones varied depending on the type of birth-control pills used. “We know that American women experience more periods than the Dogon because they start menstruating earlier and have fewer children, and we know that having more periods is associated with a higher breast cancer risk,” she explains, noting that the relation is likely because of the additional hormone exposures accrued from those extra periods. “But we don’t really know how that risk squares with the hormone exposure women are also getting from long-term use of birth-control pills.” After analyzing data from 12 studies, as well as the information on birth-control package inserts, Strassmann’s team concluded that some types of the pill exposed women to a quadruple dose of progestin (a synthetic form of progesterone contained in the pill), relative to the progesterone their naturally cycling body would produce.

Nobody knows for sure what that exposure to synthetic hormones will mean long term for women using the pill to suppress their cycles indefinitely. This knowledge gap speaks to broader concerns about our ignorance around menstruation. If Rock and Pincus had begun their work with a deeper understanding of menstruation’s evolution and purpose, how might that have affected the pill’s development? Would women today have more—and more targeted—options to manage their menstrual pain and associated disorders?

In this latest iteration of our menstrual taboo, dispatching with the period instead of researching its complexity might have unforeseen health consequences, Prior says. “Our data on the pill come from generations of women who followed the schedule for 28-day cycles and didn’t stay on it for nearly as long as women do today,” Kissling says. “What we have now” with women using birth control for long-term suppression “is the largest uncontrolled medical experiment on women in history.”