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

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.