Did you know that women have on average about 450 periods during their lifetime? That may seem like a big number, but despite it being such a common occurrence, many women don’t know what’s going on during menstruation.

Historically, menstruation has been taboo, and thus the topic is often misunderstood. Today, we still find many women who are embarrassed to talk about this natural bodily function. Because of this shame, the knowledge surrounding periods and, to a wider extent, female sexual health, is lacking. This needs to change.

This book summary dissect the historical taboos about discussing periods and provide you with answers to basic questions, such as, “Is period sex normal?” and “Why do periods even exist?”

With greater insight into what’s normal and what isn’t, you’ll be able to make more informed choices about your body.

In this summary of Flow by Elissa Stein and Susan Kim, you’ll learn

• why women experience cramping during their period;
• what women did before tampons and pads; and
• how drug companies use outdated perceptions to sell their products.

Before the age of scientific knowledge, myths provided explanations for why young girls and women bled from their vaginas on a monthly basis. Though these stories portrayed menstruation as a powerful process, ancient people also perceived it as a marker of women’s inferiority. Thus, period blood was simultaneously understood as a sacred substance of life and a toxic matter.

So, though they often believed that this vaginal bleeding was the sacred remains of an unborn child, ancient peoples also condemned it as evil and dangerous.

According to Roman philosopher Pliny the Elder in his book Natural History, written in AD 77, period blood could cause a horse to have a miscarriage and the extermination of flowers, among other things. These assertions remained uncontested for more than a thousand years. Furthermore, the belief that period blood is toxic persisted well into the twentieth century. Even today, certain cultures still believe it.

Based on the ancient belief that menstruation is the process of the body cleansing itself from the toxicity of menstrual blood, doctors developed a procedure called bloodletting. Bloodletting was a process in which illnesses were treated via the draining of blood from a vein.

Bloodletting was used on both men and women. But since menstruation was a wholly feminine phenomenon, the myths and misperceptions surrounding it were used to subvert women’s position in ancient society.

Back then, a woman on her period would have to go away to a menstrual hut. Unbelievably, this arcane act still exists in some parts of the world. Not only that, but menarche, or the onset of menstruation, would be followed by rituals. One such ritual in British Columbia forced girls out into the wilderness; one in New Ireland kept young women in cages for up to four years.

Menstruation was also used as an excuse to exclude women from different types of institutions. Even in the 1920s, for example, menstruating women weren’t allowed to enter churches around the world, wineries in Germany or opium labs in Vietnam.

Today, menstruating women are banned from partaking in Islamic rituals. These outdated beliefs surrounding periods have had a significant effect on contemporary societies all over the world. We’ll explore this more deeply in the upcoming book summarys.

Throughout the Middle Ages, women exhibiting signs of “hysteria” were accused of being witches. These signs included anything from insomnia to random bursts of laughter or crying. In modern times, however, the same symptoms of hysteria are referred to as premenstrual syndrome or PMS.

The diagnosis of hysteria had a great impact on women’s history, illustrating the general ignorance of female anatomy and sexuality. For example, even the “Father of Medicine,” ancient Greek physician Hippocrates, believed that hysteria was caused by the uterus snaking its way around a woman’s body. The term was only dropped by the American Psychiatric Association in 1952, and the diagnosis of premenstrual syndrome developed a year later.

Treatment of hysteria involved everything from X-rays to using leeches on the patient’s vulva. The most common treatment, however, was a doctor or midwife stimulating the patient’s clitoris until they reached orgasm. As crazy and sexual as this sounds, it was viewed as a wholly medical procedure and was not to be performed at home unsupervised.

Similarly, today’s understanding of PMS is as unclear as the outdated diagnosis of hysteria.

While we know some things about PMS, like how cramps are the result of the uterus contracting, there is a lack of research on the causes of other symptoms, such as insomnia or mood swings. In fact, there’s no conclusive evidence showing that PMS is hormonal! Furthermore, PMS, and the more severe condition PMDD (Premenstrual Dysphoric Disorder) are largely Western medical concepts.

Whether hysteria or PMS, one key question remains: should emotional and physiological reactions during menstruation be treated as problems that need solving or are they simply part of being human?

Of all the many taboos related to menstruation, the biggest is probably period sex. Though it may be a messy activity, period sex has not been found harmful by modern science. By educating ourselves about where these taboos originated, we can begin to unpack the myths about sex and menstruation and start challenging the stigmas surrounding them.

Historically, most religions held the dogma that period sex led to contamination, and that women required cleansing after menstruation before they could have sex.

In Orthodox Judaism, for the two weeks covering and following a woman’s period, she is deemed unclean. In these 14 days, the woman is considered a niddah, during which time she and her husband aren’t allowed any physical contact, including sex. If there is no further trace of blood seven days from the onset of her period, the woman can bathe in a ritual bath – a mikveh – to cleanse her body.

Orthodox Judaism is not alone in its harsh treatment of menstruating women. With the exception of Buddhism, many religions around the world share the same mistrust toward periods.

Muslim women are forbidden to have sex with their husbands, fast or handle the Koran during that time of the month. The Bible discourages men from shaking hands with a woman who’s on her period. What’s more, all tampon ads were banned from Polish television networks during Pope Benedict’s visit in 2006.

But despite the widespread negative viewpoints that persist in modern society, the fact is that period sex is completely safe and normal.

A 2002 study at Yale University even suggests that women who experience orgasms while on their period are less likely to suffer from endometriosis, a painful disorder where the uterine lining grows outside the uterus.

Unfortunately, this isn’t talked about enough. And, for many women, discussing anything period-related still causes feelings of deep shame.

Can you imagine what it was like for women before pads and tampons were commercially available? Having a rag strapped between your thighs for roughly seven days a month, every month, would’ve made it challenging enough to walk, let alone fully participate in societal events and activities. Therefore, the development of femcare products accompanied historic political change for women.

It wasn’t just the lack of tampons that was a challenge before femcare products were invented, it was also the lack of underwear. Until the twentieth century, underwear such as petticoats and shifts weren’t worn for hygiene reasons but rather for warmth. Some women from wealthy families were afforded the luxury of wearing rubber-lined aprons and bloomers to stop their period from tarnishing their dresses, but most women had to resort to moss, rags, leaves and sheepskin.

In 1920, Kotex pads, which were made from the cellucotton leftover from WWI bandages, became available. In the same year, the Nineteenth Amendment was enacted, allowing women to vote. Although the pads were a great advancement, they were still bulky and had to be held together with elastic belts and pins, making them highly uncomfortable.

Later on, in the 1970s, self-adhesive pads were sold by brands like Carefree and Stayfree. This coincided with the women’s liberation movement, where women were fighting for their rights on issues such as equal pay and abortion.

Today, in the West, menstruation no longer gets in the way of a woman’s education or career. But over in Sub-Saharan Africa, for example, some young girls are absent from school 10 to 20 percent of the time because of a lack of femcare products.

Thanks to television, magazine and billboard advertisements for femcare products, periods have stepped out of the shadows and become more visible than ever. But the spotlight shone on periods has only helped to serve the outdated viewpoint that menstruation is shameful and needs to be hidden.

The collective consensus on menstruation was built on successful marketing campaigns led by the two-billion-dollar femcare industry.

Femcare ads typically show beautiful women dressed in little clothing and surrounded by tranquil landscapes. No mention or reference to period blood is made. This suggests that menstruation is impure and that women require cleansing afterward.

Furthermore, though the ban on femcare ads was lifted by the National Association of Broadcasters in 1972, menstrual blood is still being represented by a blue liquid, suggesting that periods are too unclean to be seen by American audiences.

In addition to exploiting women’s insecurities about their periods, marketing campaigns are now exploiting fears about vaginal odors to sell douches.

Jokes about vaginal smells have persisted for years, creating deeply rooted insecurities for many women. The fact is, healthy vaginas usually have no odor, and if they do, it’s a sign that maybe something isn’t right and you should visit a healthcare professional.

However, since the 1930s, the femcare industry has used ads to convince women to buy products that mask these non-existent vaginal odors, claiming that these treatments could solve marital problems. They were very dangerous: popular vaginal douches such as Zonite (a weaker concentration of bleach) and Lysol (the very same chemical you use to disinfect your kitchen), or even a mixture of water and vinegar, can disturb your body’s pH levels and cause bacterial vaginosis, yeast infections and, worse yet, pelvic inflammatory disease.

Despite the proven ineffectiveness and risk of vaginal douche products, 20 to 40 percent of American women report regularly using them, showing just how deeply rooted these insecurities are.

We’re all aware of the implied meaning behind the words “I’m late,” either having heard it on a TV drama or from a good friend. What many people, including women, don’t know is that a late period isn’t necessarily indicative of pregnancy. It could indicate very different things like stress, perimenopause or even that you’re still ovulating.

The absence of information surrounding menstrual cycles and pregnancy is a result of the collective shame manufactured and exploited by American culture when it comes to anything period-related.

For example, one overlooked fact is that women are able to menstruate without ovulating – and vice versa. In the two years following the onset of menstruation, up to 80 percent of menstrual cycles don’t include ovulation. In contrast, amenorrheic women who have gone for months without menstruating sometimes get pregnant because they were ovulating despite not having their periods.

Furthermore, you can become pregnant even if you’re having sex on your period. Though many animals usually only have sex when they are in estrus – in heat – human beings have sex at any point of the female’s menstrual cycle. However, many people are unaware that having unprotected sex in the weeks following your period exposes you to a high risk of pregnancy. This is because you are most fertile after your period, during ovulation.

Even fewer people know that sperm can remain in a woman’s vagina for a few days, so if you have sex while on your period, there’s a possibility that you can still get pregnant. Additionally, the heavy spotting women sometimes get when ovulating is often mistaken for their period.

Ultimately, there is a large gap in our knowledge surrounding menstruation. This is mainly due to the distribution of information by big businesses that are more interested in selling their products than they are in educating women about their natural bodily processes.

Did you know that over 100 million women from all over the world use the birth control pill? Surprisingly, most of them aren’t aware that the bleeding they experience while on the pill isn’t their period. It just goes to show how many women don’t know enough about menstruation.

Let’s begin at the top. What is a period?

When your cycle starts, your brain messages the egg follicles that are located in your ovaries to begin ripening. On the thirteenth day, or thereabouts, the brain sends out another signal to let your body know it’s time to ovulate. The remaining follicle (or follicles in the case of twins) opens and releases the hormones progesterone and estrogen, which tell the uterus lining – or endometrium – to expand so that the egg can travel through the fallopian tubes to the uterus.

If the egg encounters any sperm on its journey, it will latch itself onto the uterine wall. Here, it will form a placenta, which will be nourished with blood via three arteries from the uterus.

If there is no conception, the egg will simply dissolve, and the arteries will stop pumping blood to the uterine wall, effectively killing the layers of endometrial tissue on the uterine wall. Then, to get rid of the tissue, the arteries open up for a short period to let the sudden pressure of blood rush through and rupture the layers. During this process, gravity and uterine contractions – more commonly known as cramps – accelerate the passage of the dead tissue through the cervix and out through the vagina. On the outside, what you’re left with – and what period blood is made of – is blood, uterine tissue and mucus.

However, what happens when you’re on the pill is something else.

Typically, birth control pills contain estrogen or a mixture of estrogen and progestin, which you take for three weeks to trick your body into thinking that it’s pregnant. During the remaining week, you take placebo pills, which create a mild version of endometrium that breaks down and exits the vagina.

Therefore, you experience gentler versions of blood flow, cramps and other related symptoms when you’re on the pill because it’s not really menstruation at all.

Have you ever tried to imagine life without periods or the side effects of menopause? Seems pretty great, right? But if you think about it a little more deeply, does it sound like it’s a good idea to have these bodily processes eliminated for good?

To get right to the point, it’s a sales pitch from drug companies. Remember the birth control placebo pills mentioned in the previous book summary? They were originally included in birth control pill packages because drug companies feared that women wouldn’t buy something that suppressed a natural bodily function.

However, today, we can stop or reduce our periods with drugs such as Yaz, Lybrel and Implanon, which are basically birth control pills without the placebo.

Understandably, women who suffer from extremely painful cramps and other menstrual symptoms might view these period-restricting drugs as a more tolerable alternative to having a hysterectomy (the surgical removal of the uterus). But the truth is, most women don’t struggle much or at all with the symptoms of menstruation and so these drugs don’t deserve to be promoted as a worldwide “solution.”

As with periods, pharmaceutical companies have also tried to exploit women’s fear of menopause. For a long time, menopause was seen as a disease. Nowadays, we understand that it’s nothing more than a natural process that female bodies undergo. With the average life expectancy as high as it’s ever been, more and more women are going through it.

But despite growing exposure, women still fear the symptoms that accompany menopause, such as hot flashes, difficulty concentrating and mood swings, as well as signs of aging. Drug companies have capitalized on the fear of aging since the 1930s, convincing women that they can avoid it with Hormone Replacement Therapy (HRT). Though long-term use of HRT has been known to increase the risk of blood clots, heart disease, stroke and breast cancer, drug companies still insist on marketing these drugs to consumers today.

The key message in this book:

For centuries menstruation was demonized, and people were afraid to discuss it in public. By exploring the historical and cultural context surrounding periods, we can begin to open up the discussion and arm women with knowledge about their natural and normal bodily processes.

Actionable advice:

Start a period diary.

Record all the information about your cycles, either in a physical diary or digitally with an app. The best way to learn more about what’s happening to your body every month is to be attentive to it. Make a note of whether you’re feeling tired or emotional, hungry or bloated, whether or not your period was on time, when you had unprotected sex, and so on. Having a record of your body’s changes will help you keep track of everything that’s going on down there so that there won’t be any more surprises or freakouts in the future.