Why do women have periods?

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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 mammals on Earth that menstruate, and we also do it more than any other animal, even though it’s a waste of nutrients, and can be a physical inconvenience. So where’s the sense in this uncommon biological process? TED-Ed describes the history and evolution of menstruation.

Lesson by TED-Ed, animation by TED-Ed.

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 its a waste of nutrients
and can be a 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 blood stream.

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 fetus's 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 implantation,

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.