
Why can’t women get pregnant without the menstrual cycle? (2016) - johnny313
https://www.quora.com/Why-do-women-have-periods-What-is-the-evolutionary-benefit-or-purpose-of-having-periods-Why-can%E2%80%99t-women-just-get-pregnant-without-the-menstrual-cycle/answers/4625918?share=1
======
madethemcry
I first wondered if OP means that first answer with the obscure and horrible
programming analogy then I found the real answer by Suzanne Sadedine which is
an amazing read.

That was the creepiest part:

 _Some fetal cells find their way through the placenta and into the mother 's
bloodstream. They will grow in her blood and organs, and even in her brain,
for the rest of her life, making her a genetic chimera_

and this is a wonderful summary of evolution:

 _In other words, it 's just the kind of effect natural selection is renowned
for: odd, hackish solutions that work to solve proximate problems_

~~~
Terr_
I came across this essay a while back titled "War in the Womb" which brings up
game game-theory components to the two systems "fighting".

[https://aeon.co/essays/why-pregnancy-is-a-biological-war-
bet...](https://aeon.co/essays/why-pregnancy-is-a-biological-war-between-
mother-and-baby)

~~~
Ygg2
I doubt it's all black and white as you tell it:
[https://www.newscientist.com/article/dn21185-fetus-
donates-s...](https://www.newscientist.com/article/dn21185-fetus-donates-stem-
cells-to-heal-mothers-heart/)

If the mother's body leaves fetal cells alone, they have can act as
essentially free stem cells, helping repairs.

I forget the exact journal, but women that had like two or three kids, live
longer on average than those that didn't.

~~~
ajuc
> women that had like two or three kids, live longer on average than those
> that didn't

Can't have 3 kids if you died at age of 10.

~~~
mtgex
While that's true, the context of the statement makes it clear that's not
what's being discussed at all.

------
abhishek0318
Perhaps, linking this post to this link would be better:
[https://www.quora.com/Why-do-women-have-periods-What-is-
the-...](https://www.quora.com/Why-do-women-have-periods-What-is-the-
evolutionary-benefit-or-purpose-of-having-periods-Why-can’t-women-just-get-
pregnant-without-the-menstrual-cycle/answers/4625918)

~~~
dang
Ok, we changed to that from [https://www.quora.com/Why-do-women-have-periods-
What-is-the-...](https://www.quora.com/Why-do-women-have-periods-What-is-the-
evolutionary-benefit-or-purpose-of-having-periods-Why-can%E2%80%99t-women-
just-get-pregnant-without-the-menstrual-cycle?share=1). Thanks!

------
diN0bot
perhaps the most gripping read of any hacker news post ever. seriously
entertaining and informative!

~~~
lordleft
Seriously! This was unbelievably compelling!

~~~
tomp
I wish I had teachers like this. I found most of history, most of my primary
and high school (i.e. several different teachers) excrutiatingly boring. Then
I read a book on the _economic_ history of the world that didn't just present
raw facts, but also why things happened (i.e. stories) and it was one of the
most interesting things I've ever read!

~~~
hi41
Which book on economic history was that?

~~~
tomp
I've no idea. It was short, probably less than 50 pages, covered the period
since about 1500. A friend sent me a .pdf but I can't find that email, or the
pdf, I've tried searching before.

~~~
hi41
Yes, I know that feeling. I had This is Water essay by David Foster Wallace in
an email and I couldn't find it and the page I had bookmarked was gone too. I
was so upset. The essay keeps coming back in my thoughts.

------
jessaustin
Mice and rats are an interesting case, since they have hemochorial placentae
but not menstruation. This somewhat undercuts TFA's "this because of that"
narrative. (Really the only flaw in a wonderful article.) Evolution is weird.

~~~
mbroshi
Apparently some mice do menstruate[1]. Do mice have another mechanism for
handling the issue the author raises: "What to do when the embryo died or was
stuck half-alive in the uterus?"

[1]: [https://www.nature.com/news/first-rodent-found-with-a-
human-...](https://www.nature.com/news/first-rodent-found-with-a-human-like-
menstrual-cycle-1.20072)

~~~
jessaustin
I would speculate that it works similarly to most other mammals. Spikes of
prostaglandin in the blood cause abortion via smooth muscle action and fluid
production (or, if the embryo has developed to term, parturition rather than
abortion). Prostaglandin is produced in the body cyclically. The corpus luteum
releases progesterone that counteracts prostaglandin, but toward the end of
the estrous cycle the CL degrades into an inert corpus albicans. Healthy
implanted embryos release gonadotropin, which "stops the clock" for the CL so
that it continues to produce progesterone. Eventually the placenta starts
producing its own progesterone, and at that point the CL is no longer
important. A dead or half-alive embryo is likely to produce neither
gonadotropin nor progesterone, so it will be aborted and flushed on the next
prostaglandin spike. The later this occurs in the pregnancy, the more stress
is placed on the mother.

It's my impression that after implantation, this process is the same in humans
as in other placental mammals. Thus menstruation might be more properly
understood to be about flushing unimplanted zygotes rather than aborting
unhealthy embryos.

------
blattimwind
Even Wikipedia describes placentation like a siege:

> The trophoblast, which is a collection of cells that invades the maternal
> endometrium to gain access to nutrition for the fetus, proliferates rapidly
> and forms a network of branching processes which cover the entire embryo and
> invade and destroy the maternal tissues. With this physiologic destructive
> process, the maternal blood vessels of the endometrium are opened, with the
> result that the spaces in the trophoblastic network are filled with maternal
> blood; these spaces communicate freely with one another and become greatly
> distended and form the intervillous space from which the fetus gains
> nutrition.

------
csours
>"Researchers, bless their curious little hearts, have tried to implant
embryos all over the bodies of mice. The single most difficult place for them
to grow was – the endometrium."

Whoa...

~~~
astrodust
It's the thunderdome, but not as bad as a shark uterus. They generally only
give live birth to one shark because that shark has eaten all the others.

~~~
nasredin
And sharks including threshers, makos and great whites have an even odder
solution: They feed their fetuses an endless stream of unfertilized eggs.

2\. Sibling cannibalism

Sand tiger sharks have the most ghoulish fix for hungry babies: The fetuses
devour each other in-utero until there’s just one pup remaining in each of
their twin wombs. This uterine bloodbath is thought to have developed because
a litter of sand tiger pups usually has multiple fathers — and each dad is
engaged in a proxy war for his genes to take the upper hand. The “winning”
fetus gets all the nutrients from its siblings and a roomy womb to develop
into an unusually big baby, over 1 meter (3 feet) long at birth. Being born
large and well-developed helps these newborn predators survive in a hostile
ocean.

------
teacpde
> _Normal mammalian pregnancy is a well-ordered affair because the mother is a
> despot. Her offspring live or die at her will; she controls their nutrient
> supply, and she can expel or reabsorb them any time. Human pregnancy, on the
> other hand, is run by committee – and not just any committee, but one whose
> members often have very different, competing interests and share only
> partial information._

Such eye-opening and fascinating analogy.

~~~
grzm
Thanks for the quote. That said, please don't use indentation to set off block
quotes, as it makes it very difficult to read, especially on mobile devices.

Common methods of block quoting on HN is prefixing with ">" and possibly using
asterisks to italicize the text, and possibly wrapping it in quotation marks.

~~~
teacpde
Thanks for the tip, I checked it up on mobile, it's indeed hard to read.
Should be better now.

------
DogOfTheGaps
Fascinating. Reminded me of the _The Selfish Gene_ , especially the part
discussing the opposing objectives of the mother and offspring.

~~~
Ygg2
The Selfish Gene isn't wrong, but its not true either. It's true your genes
exist in you, and they generally evolve to survive.

However, "Selfish" genes can do some rather unequivocally selfless things.
E.g. symbiosis.

~~~
lukas099
Symbiosis isn't exactly selfless; it benefits both organisms involved.

Even if you mean altruism though, a gene for altruistic behavior can still be
selfish. For example, if Gene X causes an organism to sacrifice something for
another organism that carries also carries Gene X, the gene will be promoting
its own survival.

~~~
Ygg2
It's not exactly selfish either. You realize there is a benefit to
cooperation. Most cell organelles are product of symbiosis.

~~~
mtgex
lukas099 is arguing one of the central points of the book, that altruistic
behavior is inherently selfish on a genetic level.

You seem to be arguing completely based on the title of the book, which itself
makes clear that the title is provocative but not entirely accurate, which
makes me think you have not read it.

~~~
Ygg2
I am aware of that, and I'm arguing, that is a very reductive model of how
humans behave.

E.g. when given choice, humans tend to help each other much more than a
selfish gene model would behave, i.e. help people and strangers you have no
connection with.

~~~
mtgex
But all of that is discussed at length in the book we're talking about. To
pick out the word "selfish" and say that that's the gene model displays to me
that you haven't read the book.

It's difficult to defend a book's premise from someone who hasn't read it and
just wants to debate the meaning of the title.

~~~
Ygg2
Look, I'll be honest. I didn't read a book.

But I don't want to debate its title, instead its reductionist view of people
nothing more as vehicles for the immortal DNA.

Anyway here is the part that I was referencing:

    
    
        The self-selecting process predicted by the selfish-gene model 
        becomes quickly skewed when correlations in reproduction
         exist which give rise to less than complete mixing of alleles 
        in the gene pool. This may occur through several mechanisms,
         including mate selection and partial geographic isolation.
    
        The gene-centered view, Dr. Bar-Yam points out, can be  
        applied directly only to populations in which sexual  
        reproduction causes complete allelic mixing. (Such 
        populations are called "panmictic" in biology.)
    
        Many organisms are part of populations that do not 
        satisfy this condition. Thus, the gene-centered view and 
        the concept of the "selfish gene" does not describe the 
        dynamics of evolution, Dr. Bar-Yam concludes.
    

Source:
[http://www.necsi.edu/projects/evolecol/selfishgene.html](http://www.necsi.edu/projects/evolecol/selfishgene.html)

~~~
lukas099
>Look, I'll be honest. I didn't read a book.

>But I don't want to debate its title, instead its reductionist view of people
nothing more as vehicles for the immortal DNA.

The problem is that the book does not expound this reductionist view; it is
only your conception of the book (likely based on the title) that does.

~~~
Ygg2
Except it absolutely does:

    
    
        We are survival machines – robot vehicles blindly
        programmed to preserve the selfish molecules known as genes.
        This is a truth which still fills me with astonishment.
    

Richard Dawkins, The Selfish Gene.

~~~
mtgex
As lukas099 has said, the book does not take the view that you're saying it
takes.

Cherry picking quotes out of the context of an entire book can get you to any
conclusion you want to come to, especially if you don't know the rest of the
contents of the book.

I encourage you to read the book before taking a side that you read an article
about.

~~~
Ygg2
You still are dodging addressing my points, before going into a "READ THE
BOOK" mode. I don't have to read the Bible to realize how it was made and why
is it flawed... Similarly, I don't need to read the book, to know the
inclusive selection theory on which The Selfish Gene was based is flawed, and
hasn't lived up to its models.

EDIT: Started reading it:

    
    
        The replicators that survived were the ones  that built survival  machines  for themselves   to  live in...
        We  are  all survival  machines  for the  same kind of replicator—molecules called DNA—but there  are many...
    

My former quote sure is misinterpreting things /s

------
gugagore
If I read anything about human reproduction, I remember "The Egg and the
Sperm: How science has constructed a romance based on stereotypical male-
female roles" by Emily Martin.
[https://web.stanford.edu/~eckert/PDF/Martin1991.pdf](https://web.stanford.edu/~eckert/PDF/Martin1991.pdf)
one quote in relationship to the comparison between ovulation and
spermatogenesis: "By extolling the female cycle as a productive enterprise,
menstruation must necessarily be viewed as a failure."

------
danieltillett
While the post was very entertaining, it did not actually answer the question.
We don't know why higher primates (and bats) have periods.

~~~
bpicolo
> The solution, for higher primates, was to slough off the whole superficial
> endometrium

Doesn't that suggest that's reason for higher primates? Or are you disagreeing
with the author of that post?

~~~
danieltillett
This is the hypothesis, but no evidence to support this theory is advanced. We
don't know that higher primates (and bats) have more aggressive embryogenesis
than other animals that don't have periods.

------
teekert
Did anyone get a pop up on the page saying the page was "reestablishing
internet connection, click here". I clicked but immediately felt pretty
stupid, I never just click on things but this was so... normal. And why would
a page and not the browser report this?

------
lazysheepherd
Same story illustrated by TED-Ed, which I would suggest you to watch:
[https://youtu.be/cjbgZwgdY7Q](https://youtu.be/cjbgZwgdY7Q) I happen to watch
it few weeks ago and this reminded me of it.

------
photonios
What a brilliant piece of writing! Very compelling.

------
nishagarg
Ummm.. someone I know did not have the menstrual cycle (had for only 2 initial
years) but she still has 2 kids healthy and fit.

------
letientai299
After reading this, I just miss my mother so much. Can't imagine what I've
done since I'm still a fetus.

------
cryptonector
Shorter version: menstruation probably only exists to potentially flush failed
pregnancies.

------
diedyesterday
TL;DR => Menstruation in human females is a monthly self-cleansing (although
wasteful) FLUSH to get rid of all those "unworthy" scum who didn't make it as
viable fetuses. Because being considered "worthy" by mother grants you (the
fetus) root-level, administrator access to her body (Yes, we human fetuses are
that aggressive and we don't know why!). So be warned that there is a tough
screening process going on before you get accepted and gain root-level
permanent access!!! Otherwise you will end up in nasty menstrual blood!

------
randyrand
this BBC article disagrees:

[http://www.bbc.com/earth/story/20150420-why-do-women-have-
pe...](http://www.bbc.com/earth/story/20150420-why-do-women-have-periods)

~~~
nostrademons
It actually supports pretty much all of Dr. Sadedin's details if you read past
the first half of the BBC article, which describes previous discredited
theories.

------
tetrazine
Very interesting answer from Suzanne Sadedin. Very odd that "Arush Kakkar,
Founder, CorsecoTech", who appears to neither be a physician, nor a medical
science expert, nor a woman, is the highest displayed answer for me, with a
strained analogy relating women's bodies to software, which is based on
Suzanne's answer. I don't know if this is because of user voting, his score,
or the Quora algorithm. I wonder if the mysteries of this ranking system are
what lies behind the Quora login wall.

~~~
elil17
Sadedin's post has over 20k upvotes, but that guy has 81. Definitely not
because of user voting.

~~~
nostrademons
It's fallen to #2 now. I would bet it's recency bias; that guy is probably
there from this Hacker News link, made a follow-up answer, and because Quora
(like HN and many forums) tends to put new replies at the top to give them a
chance & see how they do with voting, it momentarily showed up as the top
answer.

~~~
raldi
Quite the opposite; his answer was from Jan 2015; the good answer arrived
nearly two years later.

~~~
nostrademons
I'm not terribly familiar with Quora's UI, but it looked like that's the date
that the person's _author profile_ was last updated, not the date that they
answered the question. I could be totally wrong on this though. It's not
possible for the good answer to have arrived later, because the bad answer
references it (unless he edited the post afterwards to refer to it, but if the
date on the post is actually a last-edit date for the post, that still
wouldn't make sense).

~~~
raldi
Oh, I think you’re right.

------
cornholio
Artificial wombs can't happen soon enough, it's the final challenge to equal
rights.

------
simonblack
Strictly speaking, they can. The period occurs at the end of a cycle, so it's
possible to get pregnant on your very first cycle without ever having had a
period.

The likelihood of this happening is extremely low of course.

~~~
imron
Strictly speaking, the question is asking why women can't get pregnant without
the menstrual _cycle_. As you mention in your post, the period occurs at the
end of the cycle, therefore without a cycle, pregnancy will not happen.

~~~
mirimir
No. Menstruation occurs if there's no implanted and viable fetus by ~24 days
after ovulation. A few days later, ovulation occurs, and the cycle repeats.

~~~
newfoundglory
What are you saying no to?

~~~
mirimir
Maybe I read it wrong. Pregnancy and menstruation are alternate endings for
the cycle. So, as GP said, a woman could theoretically get pregnant on her
first cycle, without menstruating.

~~~
newfoundglory
I think you are agreeing with the comment you responded to.

~~~
mirimir
Maybe so. Too late to delete.

