Why do humans – and so few other animals – have periods?

Image credit: Patricia Moraleda via Pixabay

Opposable thumbs, the power of speech, the capacity to think and reason: there are many reasons to relish being human. Regularly bleeding out of your vagina is not one of them. 

We are taught that menstruation is a normal part of the reproductive cycle, a necessary by-product of being a sexually reproducing species. Every cycle, before an egg is released from the ovaries, reproductive hormones like progesterone cause the lining of the uterus – the endometrium – to thicken and fill with blood vessels. If the egg is fertilised, the uterus is ready to receive it so that it can implant and start growing. If not, progesterone levels fall rapidly, and the endometrium begins to shed. Your regularly scheduled underwear-staining, cramp-inducing crimson tide has arrived. 

It turns out, menstruation is quite rare in the animal kingdom, even amongst mammals. Other primates menstruate (though not as heavily as humans do), as do some species of bats and elephant shrews. That’s it. The number of myths, taboos and misconceptions surrounding this phenomenon are a testament to how rare and alarming it is  – the word taboo itself likely derives from the Polynesian word for menstruation, tapua. Pliny the Elder explained that menstrual blood could stop seeds from germinating, wither plants, and make fruit fall from trees. Conveniently, it could also clear a field of pests if the menstruator walked around in it naked. Throughout history, people have been forced to isolate themselves during their periods, for fear they would spoil food or cause men to lose their virility. Still today, menstruation is poorly understood because periods are widely stigmatised and talking about them openly is uncomfortable. A recent survey from the makers of the cycle-tracking app Clue found over 5000 euphemisms for the word “period”. Among my favourites: ペリー来航, Japanese for ‘the arrival of Matthew Perry’ – the naval commander not, sadly, the Friends actor; les anglais ont débarqué, French for ‘the English have landed’, in reference to the red coats the English wore in the Napoleonic wars; and, inexplicably, kommunister i lysthuset, Danish for ‘communists in the gazebo’. 

Menstruation seems like such a wasteful, energy-consuming process. It would be like deep cleaning your spare room every month for a guest who might not arrive, and whom you might not even want in your house. And yet, menstruation evolved independently at least three times, so it must have some evolutionary advantage. 

Pregnancy is often viewed as a magical time in a person’s life, during which the birth parent forms an intimate bond with their child. In fact, pregnancy is an all-out evolutionary war. On one side of the battlefield, we have the embryo, and its genes. Its goal is to divert as much nutrition as it can from the parent so that it can grow to propagate its genes; human embryos are amongst the most invasive during pregnancy. On the other side, we have the parent. The parent wants to conserve their energy so that they can have many children to propagate their genes. There is some evidence that the embryo gets some genetic reinforcements from its father, whose genes are pitted against those of the parent in the early stages of embryo development. Natural selection may have acted to advance each belligerent’s goals. 

To understand why menstruation evolved, we have to think of it as a by-product of spontaneous decidualisation. In most mammals, decidualisation – the thickening of the uterine wall – is controlled by the embryo: it occurs in response to fertilisation rather than in preparation for it. In menstruating species like humans, spontaneous decidualisation is one way the parent tries to wrest back dominance of their uterus from an increasingly invasive embryo. The uterine lining now responds only to the parent’s hormones rather than the embryo’s, and the parent controls whether or not they get pregnant. They put their defences up preemptively, by sealing off the main blood supply from the endometrium before the embryo implants there. Not content with this, the embryo evolved to burrow through the endometrium until it reaches the arteries, where it tears through the wall and rewires the blood vessels so that it can bathe directly in the parent’s blood. The (arguably) ungrateful parasite pumps out hormones to make the arteries expand around it, and paralyses them to prevent the parent from cutting off its supply. It produces more hormones, which act directly on the parent to maintain pregnancy and increase the availability of nutrients. The parent defends themselves as best they can: their endometrium fights against the embryo’s invasive proteins, their immune system attacks the invading cells, and their own hormones try to counteract those of the embryo. The tug-of-war rages on. 

Human embryos may be aggressive, but they are also particularly prone to genetic abnormalities. Genetic analysis of morphologically normal, high quality embryos during IVF showed that around 70% of embryos have complex chromosomal abnormalities such as aneuploidies. An aneuploid cell has either too many or too few chromosomes, and most aneuploidies are lethal. It’s unclear why, but the rate of aneuploidy in human embryos is estimated to be 10 times higher than in other mammalian species. Therefore, the second advantage of spontaneous decidualisation is that menstruation gives the parent the opportunity to get rid of defective embryos. As we’ve established, growing a new human takes a considerable toll on the parent. Spontaneous decidualisation gives them the power to select the embryos that have the best chance of prospering. Embryos with lethal aneuploidies are more metabolically active than healthy ones, possibly because they are expending more energy just to stay alive despite their abnormal genotype. When the cells of the endometrium prepare for a possible pregnancy, they gain the ability to sense this metabolic activity. If the endometrium decides that an embryo is unworthy, it actively hinders the migration of the embryo through the endometrium. The uterine lining is then shed, taking the embryo with it. This could explain why, compared to other species, humans are inefficient procreators: it has been estimated that only half of human conceptions progress to a full pregnancy. However, it is also thought that the repeated cycle of regeneration may allow the human endometrium to adapt and improve. In other mammals, this renewal would only occur at the end of a pregnancy, whereas humans have a monthly practice run. This could be why most people who suffer from recurrent miscarriages eventually have a successful pregnancy. 

Perhaps menstruation is particularly bothersome to us nowadays because we have so many periods. For most of human history, having a period would have been rare. Other menstruating mammals and natural fertility human populations (so-called because they don’t use contraception) spend most of their reproductive life either pregnant or breastfeeding, during which time they stop menstruating. In Tanzania, the Hadzabe people have around 6 children on average, which they breastfeed for 4 years. At most, they likely have a few tens of periods. In contrast, people who menstruate in our society can now expect to have 300-500 periods over the course of their lifetime. For such a common occurrence, it’s appalling that we know so little about it, and discuss it so shamefully. 

But now if you’ll excuse me, I have to go deal with the communists taking over my gazebo. 

Written by Helena Cornu and edited by Ailie McWhinnie.

This article was originally published on 9/4/20 and updated on 26/6/20 to use more inclusive language.

24 thoughts on “Why do humans – and so few other animals – have periods?

  1. Cara

    Loved the framing of periods and pregnancy as a battleground! Would easily read an entire book like this.

  2. Michał

    Very interesting and informative article, but unfortunately it’s excellent insights are marred by issues completely (in my opinion) unrelated to its subject matter. I have nothing against the scientific reasoning and language putting emphasis on conflict, since it makes sense, however the “inclusive” language gives me very exclusive vibes. Since the father is no longer considered a parent, why not go all the way and call him “the donor”? Why warp a language in favor of 0.0000000001% of the population, i.e. female to male transsexuals who actually want to get pregnant, and demean everyone else in the process? Such clumsy and divisive attempts at artificially forcing inclusivity into a language will yield nothing but further division.

    1. Till

      There are lots of people who don’t identify as women but aren’t trans men. Many of them will give birth to children as well. There are also many women who will give birth and not keep the child, for example: surrogates. My sister was a surrogate and was deeply uncomfortable when anyone referred to her as the mother, because that wasn’t her role in the baby’s life. It doesn’t hurt you to be more inclusive, but it can hurt people who are excluded.

    2. Lorna

      ‘the donor’ can be more than just a trans man (who would most likely be considered a father anyway). using the term ‘father’ has no relevance to this article, as the author isn’t writing about familial relationships. the pregnant person many not consider the person who impregnated them as the ‘father’ of their child (if if they are genetically the father) so using the word ‘donor’ is just more inclusive language.

  3. Casey

    I really appreciate the gender-neutral language, since not everyone who can give birth is a woman/mother. For example, trans men who give birth are called seahorse dads, referencing seahorses’ reproduction where the father carries the developing embryos.

    1. beep

      you still need a uterus to give birth.

      1. s

        Literally no one has ever said that you don’t.

  4. Zach

    Great article! So interesting to find that most mammals do not menstruate and then finding out why. Some day, I hope we can grow babies externally and cease menstruation for women so they don’t have to mess with it. I always thought that sounds extremely annoying.
    Your writing style is fantastic. Very sardonic but informative. I too would read a whole book about this so long as you write it 🙂

  5. Sofia

    Thanks for sharing,
    It’s a very informative and interesting article.
    I really enjoy your content.

  6. Biology

    Having breastfed for 9 years I can tell you that periods return in my case (and in many others given all the other women I know in this situation) after about a year. Breastfeeding does not suppress menstruation for the duration of breastfeeding which is a really well known basic fact. Plenty of women get pregnant whilst breastfeeding, breastfeed throughout pregnancy and then go on to tandem feed (amazing women that they are). Which inclines me to take anything else in this article with a grain of salt. I also find it really exclusive to not mention the word mother. It’s important for many mothers to be called mothers. You CAN say mothers and other birth parents, but it’s erasure of mothers to just exclude the 99% who identify as mothers and wish to be referred to as such. Inclusivity which excludes the majority isn’t inclusive.

    1. Nike

      I agree that lactation does not suppress ovulation and menstruation indefinitely. However, I know a bit about some of these cultures. For instance, in my local African culture, semen was thought to make the baby being nursed very ill. So, couples abstained for as long as the child was being breastfed, which serves as natural child-spacing. Men also had several wives, so they didn’t suffer much from the abstinence. My grandpa, who was a local midwife and traditional healer, taught me this; he said they believed semen “poisoned the baby”. Of course, no one believes that anymore, but it would likely have been more prevalent when “natural fertility human populations” were more prevalent.

      1. Heaven

        Well breastfeeding can stop a period if you exclusively breastfeeding, but even then it is not forever. It can only stop it for several months. I think women who get pregnant while breastfeeding may not have been exclusively breastfeeding or they were but EBF is not 100% effective.

    2. Heaven

      Breastfeeding does stop the period but only if you exclusively breastfeed and even then it is not 100% effective. No one is saying the breastfeeding stop your period forever. It doesn’t. Not even exclusively breastfeeding stops your period forever. You can’t use anecdotal evidence to disprove something. There are women might have different experience from you and those other women you mention.


    3. Emmy

      “Inclusivity which excludes the majority isn’t inclusive.”

      This is an odd take. Nobody is excluded by the use of the word parent. Nobody else is specifically named either, like surrogates who don’t wish to be called mother, those who give up for adoption etc. It’s a little like saying “we need to say asian people and other humans” in order to include them. We don’t and it’s odd phrasing.

  7. Denise B.

    Ah, bloody hell. To be female is to be real intimate with blood. Those who wish to be female, be careful what you wish for.

  8. Victoria Poyner

    That was an awesome educational article and the vocabulary very accessible, I would love to read more by this author and their sense of humor is just like mine. After cervical cancer and a hysterectomy I do not miss my uterus or period.

  9. Nancy

    In this cult of death it is cold and indifferent to refer to a new life as an aggressive parasite. The miracle of fertilization and birth should be looked upon in a more sacred and beautiful way, as nature’s way, and not be referred to as merely a biological occurrence.

    1. Dorthy

      There are many creatures on earth whose females do not survive pregnancy, and humans can unfortunately be one of them. To treat pregnancy as a ‘sacred miracle’ in academic and political circles would sentence many to death.

  10. Joan

    From what I understood from the writing, nature ripped females off.
    Despite the monthly pains, our embryos are still prone to aneuploidies. But other mammals who never undergo monthly pains, have embryos that are not so prone to aneuploidies.
    And I also noticed that any evolution for reproduction usually takes place in the females’ or birthing parent’s bodies.
    And it’s so painful that pregnancy is a thin line between life and death (sorry to sound so grisly) because the birthing parent is immuno-compromised.

  11. Emily

    What is wrong with people? Do you hate being a woman THAT much that you reduce the creation of a person to a war against a parasite? Messed up.

    1. Lisa Conlon

      No, it is just a basic fact of nature. I myself find it fascinating that humans have evolved so differently to other mammals, you don’t have to “hate” being a woman to realise what a pain menstruating can be. I suffered terribly during mine and ended up with massive fibroids, I couldn’t leave the house because of heavy bleeding and pain.
      I had a hysterectomy and am now pain-free thank goodness, I don’t hate being a woman but I hated my periods.

  12. j

    googled ‘why don’t humans absorb the egg’ and amongst lots of ‘is egg yolk bad’ results came across this article, which answered my very poorly worded question remarkably, hilariously, and gave me more ways to refer to my period. frankly i don’t know why we can’t change the phrase to mean the friends actor. the only thing holding us back is our chains.

  13. Rien

    This is, perhaps the funniest and well written article that answered all of my questions of why my body has decided to punish me today.

    I really appreciate the inclusive language, and the humorous tone to the piece, that also seems well researched and informative.

    Thank you very much!

  14. Dingus Manly McDonald

    “lyst” means “lust” and “hus” means “house” in Danish, in case someone was wondering about the strange euphemism.

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