Talking about reproductive health isn't easy even at the best of times. But we all stand to benefit when we better understand how our bodies function: their outcomes, their variation, and the possibilities.

Reading Time: 10 minutes

When my Grade 5 class went through Ontario’s reproductive health curriculum in the mid-1990s, the boys and girls were separated, and received remarkably different lessons. Along with the basics of our changing bodies, I and the girls were shown how to use condoms through a banana demonstration, and told how important protection would be when we were older and became sexually active. When the boys returned, we found out that they hadn’t been taught anything about condoms, let alone all the other forms of contraception that our side had been given, despite going through all the same mechanics of sex in their class.

Those of us in the girl group had a bit of a thrill at first, in getting to play instructor to the boys at recess. But it also didn’t escape us that we’d been given the burden of protection far earlier than they had. It was our first conscious lesson in bias around the topic of sex.

And yet, it wasn’t even the first example of bias within those classes. It would just take us longer to recognize all the other biases at play in how complex mammalian processes are oversimplified when we teach human reproduction. How menstrual cycles are given to us as rigid calendars, when they aren’t. How pregnancy terms are usually treated likewise. And let’s not forget the classic sex-ed video of the active sperm in a race to a passive egg: the way one side (male) fights and gives. The way one side (female) receives and nurtures.

End of story, right?

To be fair to our elementary-school instructors, though, even in the 1990s the scientific community had a ways to go in learning about the human reproductive system. A lot of our recent leaps in knowledge, in fact, draw from the now more widespread practice of in-vitro fertilization, which has given scientists far more insight into how many natural conceptions do not end in live birth, even among the most fertile demographics, and why.

If nothing else, the wealth of scientific discovery in recent decades should awaken us to the sheer complexity and dynamism of the human body.

The wonderful (and weird) human body

The fuller human story is quite extraordinary, though, because it might surprise you to know that most mammals do not menstruate. We humans are among a rare few, a list that currently includes some fellow primates, four bat species, the elephant shrew, and the spiny mouse. For most mammalian species, reabsorption at the end of their estrus cycles (e.g., when an unspayed dog or cat is in heat) is the status quo.

But we’re also not the most extreme when it comes to shedding reproductive materials: that dubious honor might go to gelada baboons, the females of which terminate pregnancies en masse when a new male takes control. Spontaneous termination of a pregnancy in the presence of a new male, called the “Bruce effect”, is found in other mammals, like mice, but it manifests at a whopping eighty percent for this baboon species, in which the dominant males will simply murder the offspring of other males if they come to term.

We humans are somewhere in the middle, and so the weird world of human menstruation and related fertility cycles still strongly informs the wide-ranging outcomes of our natural pregnancies. In particular, it reveals a far more proactive, dynamic, and discerning role for the uterus in the early stages of the creation of new life. And it highlights the complexity of talking about reproductive health in medical and other public spheres.

This is important information for all of us. Understanding the human physical baseline allows us to recognize what’s normal, what’s atypical, and how much natural variation exists. And the more data that underpins how we move through the world? The lower our chances of tripping over exceptions that stand to throw whole social systems into disarray.

Human menstruation: a mammalian rarity

To catch up readers of all sexes with the basic primer: menstruation occurs at the end of the female reproductive cycle in select mammalian species. It’s when the uterus sheds its lining, or endometrium, if no fertilized egg has successfully passed through the fallopian tubes (where it’s called a morula, and stays around sixteen cells for its two-to-three-day journey), entered the uterus (where it’s called a blastocyst, a hollow ball of cells growing to the hundreds), and implanted itself there.

Implantation isn’t an instantaneous process, though. It’s a literal embedding of one organism into the cardiovascular system of another, to form the placenta necessary to sustain the rest of the pregnancy. This process can start around five days after fertilization, but it has to align with a sweet spot in the endometrium’s development as a receptive base, and it takes a while to fully invade the endometrium to get the placental powerhouse going.

In other words, it’s a bit of a tight work window. And unsurprisingly, at least thirty percent of fertilized eggs do not survive this process. If the uterus doesn’t register a full implantation, it’s simply going to clear house, and take everything with it, at the cycle’s end.

Menstruation cycles vary significantly, and for many reasons. Weight, age, and stress levels will change a person’s menstrual norms, but those “norms” already invite variation by as much as nine days in terms of cycle length. This means that a person might have a 24-day cycle, from the end of one menstrual period to the next, and then a 33-day cycle, and still be perfectly within normal operating parameters.

(A lot of people are incorrectly given strict 28-day cycles as “the norm”, so if you’re using a menstruation tracker, make sure you have one that relies on more than a basic calendar.)

Pregnancy has been poorly measured for a long time, too. Only recently did the medical profession start to recognize that thirty percent of pregnancies would be more than ten days “overdue”, and that pregnancies varied naturally by as much as five weeks. (This is important because long pregnancies increase the chance of stillbirth, so it’s vital to have a clear sense of baseline risks.)

On the other end, not enough people understand that a pregnancy’s length is first measured from the start of one’s last menses. Not the end. The start. That means this date includes the three-to-seven-day window when the uterus is shedding its last lining, and also the full window of ovulation in which an egg could be fertilized. Someone who is six weeks pregnant, by medical terms, contains a life three or maybe four weeks in development. For around two of those other weeks, the patient was not pregnant at all.

As much as we humans want to quantify and categorize everything cleanly, it’s important to remember that our dynamic mammalian bodies are not so good at sticking to a schedule.

Miscarriage and resource costs

So why do we menstruate, and what does this odd mammalian behavior do for pregnancy?

The simple, and also not so simple, dominant theory is embryo screening, as given to us by a wealth of molecular endocrinology and related genetics research. In menstruating species, far more physical resources are invested in the resulting offspring, which represents a huge risk for the would-be parent. In less-invested mammalian species, the body prepares for a pregnancy (i.e., for the immunological accommodation of foreign tissues, while placing strict limits on fetal invasion of the parental system) when it gets pregnant. But in menstruating species, preparation comes first, before “conceptus”, and it operates on its own schedule.

This process is called spontaneous decidualization, and its use in menstruating species means that, on the long road from fertilization to implantation, the human body is far better prepared to expel any life not resilient enough to fully implant on the uterus’s schedule.

What might make a fertilized egg less resilient? That’s where chromosomal abnormalities come into play. Common forms are trisomy (a third chromosome latching onto what should be a pair) and monosomy (a chromosome lacking its partner). We humans are highly susceptible to these abnormalities, and as very few are compatible with life, they’re the most common cause of natural fetal death. You’re probably familiar with Trisomy 21, Down Syndrome. The only other chromosome pair that also routinely survives extreme variation is the sex-chromosome, which can handle a wide range of trisomy, monosomy, and even tetrasomy (four chromosomes instead of two). Conversely, very few newborns with Trisomy 13 and 18 live to their first birthday, always with severe struggles.

But there’s an added problem here, because menstruation, though it may well help to end less robust pregnancies via early miscarriage, is also an extreme draw on physical resources. To create an effective reproductive window, the body surges with progesterone to thicken the uterine lining for better implantation within a narrow timeframe. (Optimally restrictive and optimally nurturing!) However, this highly fine-tuned process is not only resource-intensive, but also generates new opportunities for cancer and other regulatory diseases. And that’s just the risk one bears irrespective of pregnancy as an end result.

The cost of being a menstruating species

Menstruating species are also far more at risk of serious complications when they do get pregnant, as compared to non-menstruating species.

Pre-eclampsia and eclampsia, for instance, account for ten to fifteen percent of global maternal deaths. Pre-eclampsia is caused by the outer layer of the blastocyst, called the trophoblast, failing to implant deep enough in the uterine lining, which results in restricted access to bodily resources for the growing fetus. This tug of war between placenta and parent incurs fetal hypoxia, and hypertensive damage to heart and blood vessels both within and beyond the uterus. And it can’t be fixed, exactly. The placenta needs to be detached from the uterus. Sometimes this can be achieved with a live birth, if the fetus is far enough along. Sometimes the fetus isn’t far enough along, though: slowly suffocating, failing to grow, while the parent’s cardiovascular health deteriorates to the point of death as well.

Another common event is called an ectopic pregnancy, which is when the industrious new life, still a morula (meaning “little mulberry”, for you etymology nuts), tries to implant somewhere outside the uterus. This can be really outside the uterus, including on the mesentery (the membrane holding intestines together) and the rectouterine cavity. But when it happens in the fallopian tube, its implantation (always nonviable, always fatal to itself) can lead to a life-threatening rupture of the organ and related tissues.

It is not an exaggeration to say that pregnancy always involves a competition for resources between two (or more) lives. Menstruation is a side-effect of a biological process that gives the uterus more control over which fertilized life it continues to gestate. The more resilient the implanted blastocyst? The lower the risk to parental health.

Still, it’s not a clean win to be part of a menstruating species (as anyone with a uterus can tell you). Just because the human body has taken on a high resource cost to stave off even higher resource costs doesn’t mean you’ll be spared a whole slew of other problems.

Mifepristone, the early-term abortifacient

One of the most common forms of birth control, “the pill”, is also used to counter some of the problems caused by menstruation (or, more precisely, by the whole process of spontaneous decidualization). This synthetic steroid consists of one or two hormones, progestin on its own or with estrogen, that halt the usual signalling involved in the release of an egg and the thickening of the uterine lining. Progestin also thickens the mucus in the cervix, which keeps sperm from reaching an egg even if one is still accidentally released.

Unsurprisingly, these interruptions to the reproductive cycle also alleviate many of that cycle’s usual side-effects. The pill is used to treat polycystic ovary syndrome, endometriosis and pre-menstrual cramping, period loss (due to stress, overexercise, or anorexia), persistent acne, and heavy menstrual flows. It also serves to prevent anemia and mitigate some risks of cancer.

One way that the pill functions to reduce period pain is by suppressing prostaglandins, a group of lipids that stimulate strong uterine contractions that help expel the lining. Another medication on the market, misoprostol, does the opposite. It increases prostaglandin presence around damaged or struggling tissues. It’s used to heal ulcers, induce labor, treat postpartum bleeding due to uterine weakness, and hasten along protracted heavy periods.

It’s also used as a follow-up to RU-486, or mifepristone, a pill that deprives the uterus of the progesterone needed to sustain a healthy uterine lining. Taken in a staggered, multiday sequence, mifepristone followed by misoprostol, the medications replicate the tail end of a menstrual cycle. They are recommended for use anywhere up to ten weeks after the start of one’s last menses, although their most effective window of use is within six weeks.

The mifepristone and misoprostol protocol was developed in France in the 1980s, and first FDA-approved as Mifeprex in 2000. (A generic was also greenlit by the FDA in 2019.) It’s currently permitted for medical distribution via mail in the US.

Why this matters to our discussion of natural pregnancy, menstruation, and miscarriage is that, if taken orally, these medications cannot be detected via blood tests or similar trace analyses. And there is nothing about the outcome of their use that can be differentiated from either a heavy period or early miscarriage. This is because so much of the struggle for life in the uterus happens very early on, so much so that a person on birth control is actually reducing the natural incidence of terminated pregnancies, simply by halting the production of eggs that, even if fertilized, then face a high risk of termination by the uterus itself.

The uterus is extraordinary, and absolutely contributes to the cultivation of new life.

If that life can first survive the earliest gauntlet set out by the organ itself.

The complexity of knowledge

One benefit of having a thoroughly mapped genealogy, as my family does, is that I can see in my lineage all the huge families come before: all the women who died in childbirth, and all the little ones who didn’t live long enough to get a name (or whose names were then given to the next child to survive). For long periods in human history, women had a great many children, usually starting as very young teens and spending most of their fertile years pregnant, and infant or young-child death was a common fate that spared few families.

Today, due to a staggering number of fairly recent scientific advances, people have fewer children to begin with, and fewer birthed infants die before the age of five. But technology has also allowed families to know more of the pregnancies that never came to term. For many of our ancestors, a lost pregnancy could be anything from a heavier period, to a surprise miscarriage after weeks of feeling unwell, to a later, more serious miscarriage ending in maternal death. Now, though, many parents can follow their offspring’s journey in the womb. They treasure not only the ultrasounds, but also the wealth of genetic data that helps them to understand why their creation will never draw breath, or else only draw breath briefly, and in great pain.

We can also intervene directly, as we never have before. Fetal surgeries have grown substantially in recent decades, and dangerous surrounding factors, like incompatible blood types between parent and offspring, can now be mitigated with medication. In vitro fertilization, too, is just one more way in which scientific advancement has allowed would-be parents to triumph over a uterus that favors a struggle for any fertilized egg’s survival.

This is an extraordinary period in human history, and it calls for great compassion to match the immense fruits of scientific process. We now know more than ever about how indifferent the human body is to the welfare of individual fertilized eggs. We know, too, how much the uterus, aided by menstruation, sets a high standard for viable pregnancy in its earliest days. And we know that around half of all pregnancies end early, most without the parent even knowing they’d conceived a new life at all.

But for those who believe that every conception event marks the creation of a new soul, and for every person who struggles to bring a wanted pregnancy to term, the brief journey of these fertilized eggs, then implanted zygotes, and embryos, and maybe even older, still ill-fated fetuses, is now knowable at a depth unheard of in any prior generation. The loss of these lives often has names attached, and familial stories, and medical histories of valiant attempts to intervene. They are cherished. They are buried. They are mourned.

Uterine knowledge, in other words, does not significantly change our pre-existing beliefs. It might not even stop a grieving spiritual parent from wondering why their god gave them a child that would not come to term. And it certainly won’t make a secular person any less inclined to see natural reproduction as an indifferent, highly resource-pragmatic process.

But for all of us, across the cosmological spectrum, improved reproductive knowledge reshapes our sense of possibility. As such, if nothing else, the wealth of scientific discovery in recent decades should awaken us to the sheer complexity and dynamism of the human body. There is nothing passive about how it operates in relation to the world around it. We could all stand to learn from how it struggles, too.

Class dismissed.

GLOBAL HUMANIST SHOPTALK M L Clark is a Canadian writer by birth, now based in Medellín, Colombia, who publishes speculative fiction and humanist essays with a focus on imagining a more just world.