The world I'm building relies on the fact that the whole population is sterile, and all babies are conceived in vitro. They do not have bellybuttons anymore but they still have genitals, and do not know how their ancestors used to live, how they reproduced, etc.

The sterility is not due to some disease, but done to the population : I was thinking, either to the foetus (hormons) or the babies once they are born (in taking the gonads with surgery or with an implant). It can't be with DNA modification.

If one woman was to get pregnant in this world (with an unsuccessful surgery, a wrong implant, etc.), she'd have to have her periods. But either she'll be the only one to have her period, and it'd be weird from the beginning, or almost every other women have their period too, but I need to find an explanation to why they lose blood each month with no logic explanation (for them).

The book The Children of Men by P.D James shows a society that became sterile one day with no explanation,

the woman that gets pregnant in the second half is one of the people that weren't tested for fertility, because she had a health problem (asthma if I recall). All women still had their periods with no questions, but they knew what it was.

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    $\begingroup$ It would seem a much longer explanation how they somehow changed the nature of everyone's uterus (or put drugs in the water) to prevent periods...unless you want that change to tie in with the sterility. Everyone has a useless appendix and a tailbone. Men still have nipples. We all stink in funny places. We all emit snot. Lots of weird legacy stuff on our bodies that we're just used to and don't need explanation. $\endgroup$ – user535733 Jul 10 '18 at 14:47
  • $\begingroup$ I get your point, for my it was weird to let something this "annoying". Nipples on men don't change anything for them; appendix can never burst, and tailbone is quite forgettable. But periods seem too much inconfortable to imagine a society not trying to avoid them, especially in a sterile world. $\endgroup$ – Lyzvaleska Jul 10 '18 at 14:54
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    $\begingroup$ As one whose appendix nearly burst, I can testify that it is anything but ignorable And @user535733, thanks to many years of suffering allergies, I can very clearly explain why snot isn't "weird legacy stuff." :-) $\endgroup$ – JBH Jul 10 '18 at 16:58
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    $\begingroup$ I think the point is that in the Author's world appendix (and perhaps allergies) are solved. I think we can all agree that they still afflict many in real life. $\endgroup$ – user535733 Jul 10 '18 at 17:13

It's amazing what we can explain, if we try. I mean, right now you're sitting down in a rather anatomically ill-advisable position starting at a small box full of lights, which you shout at randomly when the box of lights does the wrong thing. You do this for a remarkably large part of your life.

XKCD Comic

If something is known to be "normal," it is normal. It's funny how that works. User535733 brings up all sorts of great examples in the human body, like male nipples.

If your society is one that calls for medical reasons a. la. our Western science approach, one thing you could treat it as is blood letting. Iron in the blood is a funny thing. Once it gets there, there are very few things which get rid of it, other than losing blood. Your society might have "women's food" and "men's food," where the women's food is high in iron. If the women did not remove this iron from their body they could be harmed. The side effects of too much iron can include:

Excess iron in vital organs, even in mild cases of iron overload, increases the risk for liver disease (cirrhosis, cancer), heart attack or heart failure, diabetes mellitus, osteoarthritis, osteoporosis, metabolic syndrome, hypothyroidism, hypogonadism, numerous symptoms and in some cases premature death. Iron mismanagement resulting in overload can accelerate such neurodegenerative diseases as Alzheimer’s, early-onset Parkinson’s, Huntington’s, epilepsy and multiple sclerosis.

If your society's subsistence foods have too much iron in them, a culture with such food segregation might spring forth.

  • $\begingroup$ I didn't think this far, even with User535733 contribution. Your explanation helps me a lot to see yoru point of view. I will work with this optic. Thanks a lot! $\endgroup$ – Lyzvaleska Jul 10 '18 at 15:24

Two ideas.

1: Hyperestrogenism. https://en.wikipedia.org/wiki/Hyperestrogenism

This is common. The main reason in our society is obesity; adipose tissue generates estrogen. A morbidly obese society is very plausible. One could also have hyperestrogenism because of a tumor or other endogenous production. An interesting idea for scifi hyperestrogenism is environmental estrogens or xenoestrogens. Persons so afflicted would have the phenotype of high estrogen without the obesity or cancer symptoms - exactly how such women (and men, probably) would appear is left as an exercise for the writer.

Ladies with hyperestrogenism often do not have menses. They can have great difficulty conceiving / carrying a pregnancy. But every woman with this condition is not necessarily infertile. These are the ladies one reads about who suddenly have a baby, not knowing beforehand that they are pregnant. In real life, such ladies are often morbidly obese to begin with, so the addition of the pregnancy does not make as much difference for them to notice. They do not get their periods, so they do not miss them when they stop.

2: Retrograde menstruation. https://www.mayoclinic.org/diseases-conditions/endometriosis/symptoms-causes/syc-20354656 You can have a menstrual cycle but see no blood if it all goes backwards into your body. Sometimes this is because of scar tissue, or congenital abnormality like stenosis of the cervix; sometimes no obvious reason is found. It hurts a lot in a nonspecific way. Women afflicted by this are potentially fertile - although the barrier to sloughed uterus leaving also would present a barrier to sperm ingress. That could be worked around.

  • $\begingroup$ I have several problems with your answer: the society is build around the fact that the people don't reproduce (children are only "created" if older die), so it's no point to put a whole population of obese people, they're already sterile. For the retrograde, it may be dangerous, and you said painful: a woman with such an affliction wouldn't wait a few years before seeing a doctor with such pain. But I think that maybe my question wasn't specific enough. $\endgroup$ – Lyzvaleska Jul 10 '18 at 15:28

Your problem might be hold over knowledge from the past

But let's set the groundwork, first.

If the method used to enforce sterility was to prohibit the egg from dropping into the uterus, then the bleeding exists but has no purpose. Humanity is chock full of idiotsfree-thinkers who will fill any void presented to them (and occasionally will create the voids themselves). In other words, you'll get everything from the religious:

...and from the woman cometh all sin and thus she is unclean until her time is spent and after purifying herself in the sacred lake she shall present herself to the priests wherewith a lamb or two doves are spent upon the altar with prayer and she shall be again clean.

to the pseudoscientific:

...a woman's health, unlike a man's, is focused on her abdomen (but the man's is the heart and brain) and to keep a woman healthy she must peroidically cleanse herself of the effluvium.

So, filling in the explanation is really a consequence of your imagination and sociopolitical beliefs.

Unless there's a holdover of knowledge

If we use Earth as an example, the knowledge of that egg might last a long, long, long time. An awful lot of us know of it, even if we don't know a lot about it. The individual might "forget" things quickly, but the masses forget things very slowly. Wiping out the majority of knowledge bearers (doctors, books, and WebMD) would help tremendously, but it would still take a honking long time to completely forget about the egg. And so long as anyone remembers the egg, they'll remember why menstruation exists.

So, anything will do to explain menstruation, but setting up your story in a believable manner such that it makes sense no one remembers eggs... that might be the real trick.


Overt menstruation is actually not that commonly found, it's pretty much only found in primates (and a few others). Most animals have covert menstruation, where the lining is reabsorbed into the body instead of being shed.

how does that help, since we are talking about human-people who should have overt menstruation? Well, since you're tinkering with people anyway...

At some point your people had medical science to both identify and remove at least most of the reasons why overt menstruation was beneficial to human-folk, and the capability to modify human menstruation to covert - probably to minimize discomfort and/or messiness, though I make no claim this would actually work (might not've worked in the story, either, depending on how you want it to go). It doesn't even need to be everyone, even having a few people so modified might work out with some time for forgetting.

If this happened before your people mostly switched to artificial reproduction, and if this did solve at least some of the most obvious symptoms of menstruation, then whenever your people went around suppressing periods (and/or fertility) some or all of those with this modification might have gone unnoticed, since they were already not-showing the signs of fertility the usual ways. Well, they might even have had some signs that were overlooked in the absence of more obvious ones, like a period. Once people were using artificial reproduction regularly, any covert fertility would simply have gone unnoticed, it might even be a known "medical issue" (hormonal spikes, cyclical bodily issues) that just is not known to be related to fertility, as most of those so inflicted don't reproduce naturally anyway.

So when people were moving from mostly using artificial reproduction to removing the option of natural reproduction, some of those already modified might have not been sterilized, or the sterilization method might have not worked quite correctly due to the difference in the biological processes caused by the switch from overt to covert menstruation. Or even sometimes the sterility malfunctions, but is corrected at puberty - since bleeding will panic people who don't know why. So only those who have the sterility mechanism malfunction and also don't show signs like bleeding for some reason would slip by those watching.

Actually, it doesn't even have to be a deliberate modification, if some mutation or medical condition causes covert fertility, that is suppresses external periods, then all it would take is for someone with that condition to also have a malfunctioning sterility whatever.

It might take some careful world-building to have all that running in the background to convince readers without either info-dumping or leaving it handwavingly unexplained, but how that goes is up to you. Maybe a few mentions of people with hormonal conditions, that only later gets confirmed as the same thing your person has? underground teeny-tiny resistance to the sterilizers who can explain, eventually? Occasional vanishing of people who did have both, but got caught reproducing elsehow (while your person manages to skip detection)? Later testing (once your person is found out) finds a handful of others who had the same thing but by chance managed not to get pregnant?


Vestigiality is not an issue

The human body is full of vestigial remains from our ancestors. From the simple fact that you get goose bumps to the palmar grasp reflex to dozens more examples... all of these traits are inhereted from our ancestors, but that we no longer need.

So the fact that mentruation still happens, even though there is no apparent need for it, is not at all an issue and nothing strange. You as the author do not need to explain that any more than you need to explain that your characters are able to waggle their ears.


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