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a handful of species on earth share a
seemingly mysterious trait a menstrual
cycle we're one of the select few
monkeys apes bats humans and possibly
elephant shrews are the only mammals on
earth that menstruate we also do it more
than any other animal even though it's a
waste of nutrients and can be of
physical inconvenience so where's the
sense in this uncommon biological
process the answer begins with pregnancy
during this process the body's resources
are cleverly used to shape a suitable
environment for a fetus creating an
internal haven for a mother to nurture
her growing child in this respect
pregnancy is awe-inspiring but that's
only half the story
the other half reveals that pregnancy
places a mother and her child at odds as
for all living creatures the human body
evolved to promote the spread of its
genes for the mother that means she
should try to provide equally for all
her offspring but a mother and her fetus
don't share exactly the same genes the
fetus inherits genes from its father as
well and those genes can promote their
own survival by extracting more than
their fair share of resources from the
mother this evolutionary conflict of
interests places a woman and her unborn
child in a biological tug-of-war that
plays out inside the womb one factor
contributing to this internal tussle is
the placenta the fetal organ that
connects to the mother's blood supply
and nourishes the fetus while it grows
in most mammals the placenta is confined
behind a barrier of maternal cells this
barrier lets the mother control the
supply of nutrients to the fetus but in
humans and a few other species the
placenta actually penetrates right into
the mother's circulatory system to
directly access her bloodstream through
its placenta the fetus pumps the
mother's arteries with hormones that
keep them open to provide a permanent
flow of nutrient-rich blood a fetus with
such unrestricted access can manufacture
hormones to increase the mother's blood
sugar dilate her arteries and inflate
her blood pressure
most mammal mothers can expel or
reabsorb embryos if required but in
humans once the fetus is connected to
the blood supply
severing that connection can result in
hemorrhage if the fetus develops poorly
or dies the mother's health is
endangered as it grows a fetuses ongoing
need for resources can cause intense
fatigue high blood pressure and
conditions like diabetes and
preeclampsia
because of these risks pregnancy is
always a huge and sometimes dangerous
investment so it makes sense that the
body should screen embryos carefully to
find out which ones are worth the
challenge this is where menstruation
fits in pregnancy starts with a process
called
plantation where the embryo embeds
itself in the endometrium that lines the
uterus the endometrium evolved to make
implantation difficult so that only the
healthy embryos could survive but in
doing so it also selected for the most
vigorously invasive embryos creating an
evolutionary feedback loop the embryo
engages in a complex exquisitely timed
hormonal dialogue that transforms the
endometrium to allow implantation what
happens when an embryo fails the test it
might still manage to attach or even get
partly through the endometrium as it
slowly dies it could leave its mother
vulnerable to infection and all the time
it may be emitting hormonal signals that
disrupt her tissues
the body avoids this problem by simply
removing every possible risk each time
ovulation doesn't result in a healthy
pregnancy the womb gets rid of its
endometrial lining along with any
unfertilized eggs sick dying or dead
embryos that protective process is known
as menstruation leading to the period
this biological trait bizarre as it may
be sets us on course for the
continuation of the human race
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