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Pregnancy is a hardcore sci-fi epic of war,
peace and compromise. Babies are basically
very cute tumors that grow a completely new
organ inside a different human. Their mom’s
body graciously allows this, while the new
life saps her resources and challenges her
immune system to its limits for months.
Biology is brutal and beautiful and metal.
Let us zoom in and observe this brutal
struggle for survival – that in the best
case ends in one of the most
magical things: A new life.
Our story begins with an army of
tens of millions of sperm cells
that immediately face death. They
are on a tight timer before they
run out of energy and die. But this is
the least of their problems right now.
Like all bodily openings, the female reproductive
tract is a fortress defending against invaders.
The future mom’s body has set up deadly
barriers and puts the sperm through a
brutal process that kills almost all of them and
ideally selects the strongest and fittest one.
The first deadly obstacle is a highly acidic
environment guarded by hundreds of thousands
of guard cells. The acidity kills millions,
especially of the weaker sperm within the first
half hour, although they have a bit of protection:
Seminal fluid is alkaline and so it makes this
first obstacle a little less deadly. The survivors
reach the next obstacle, a treacherous maze full
of protein nets where millions more find
a brutal end getting stuck and lost.
Now if a woman is in the part of
her menstrual cycle where she is
ovulating , her body helps the sperm out a
bit, by releasing chemicals that guide the
way and by being a bit less hostile. Luckily
for our brave sperm this is the case today.
Only a couple hundred - less than 0.0001% of
the original sperm - pass through into the
uterine cavity, where two tunnels branch out.
Here mucus helps transport the stronger sperm
to move on, while weeding out the weaker
ones. The mom’s immune cells pick off and
devour every cell that trails behind or
takes the wrong turn. And then finally,
maybe just a few dozen sperm reach
their goal, the mighty, giant egg.
10,000 times bigger than them, it is
an industrial scale monster loaded with
nutrients and nearly 100,000 mitochondria,
the powerhouse of the cell – 50 times more
than an average cell. But the egg is
not easily conquered – the survivors
get closer and try to get in until the egg
decides to accept a single one of them.
The genes of two individuals merge. The
egg stops being part of its mother – it
is something else now. A new being
with its own agenda. Not a human yet,
but the potential to become one. Like a
thought that is not yet a word. But its
mom’s body hasn’t decided yet if she wants to
say it out loud. It will have to fight for that.
For a few days it rapidly divides and grows to a
couple of hundred cells, while traveling down to
the uterus, the part of the female reproductive
system where it will try to make its new home.
Here it begins to divide into two specialised
teams of cells - one will eventually become the
baby. The other cells are trophoblasts and
their job is to turn into a temporary organ
inside the mother: the placenta, that will
make pregnancy possible and eventually die.
This means that when you were an embryo
there was this whole other part of you,
not like a twin, even closer, maybe like a
guardian angel clone. These cells never had
the chance to become a human, their only
purpose in life was to make you exist.
In any case, the following days are the most
dangerous for the potential new life. Like a
tennis ball rolling through syrup the young
embryo moves along the uterus and tries to
take a hold. An intense chemical dialog between
two living things begins, the embryo releases
dozens of chemicals that announce its presence
and asks the guard cells of the uterine wall
to allow it to please, please, please attach
itself so it can survive. The uterus responds
with dozens of hormones and immune signals
itself. If it is satisfied with the quality
of the chemical conversation its cells allow it.
If it doesn’t the embryo will be lost and die.
This is where we begin to see that the interests
of both living beings no longer completely align
anymore. Pregnancy is a huge energy investment
so the mother’s body will eject the embryo if it
doesn’t see it as viable. The embryo on the other
hand faces a life and death situation and tries
to stay alive at any price. So it’s not asking
politely but deploying thousands of infiltration
units: Bubbles filled with genetic material, kind
of like a human virus! They sort of invade the
uterine cells and try to brainwash them, so they
help it attach itself, rather than rejecting it.
During this time one of the weirdest
features of pregnancy occurs: the
uterus provides uterine milk, not real milk
but a clear fluid filled with nutrients and
hormones that the young embryo sucks up hungrily
to gain additional energy until it gets inside.
If the embryo is allowed in, the implantation
was successful, the first critical hurdle. The
next step is to reach the blood vessels
of its mother, the only way to survive if
it doesn't want to starve. The trophoblasts
begin to clone themselves on a massive scale
and turn into different specialists. One of
them turns into a violent invader that starts
drilling into the uterine tissue like a
tiny parasitic octopus with many arms,
spreading and growing. This is a brutal
process, ordering loads of it’s mom’s uterus
cells to destroy themselves, killing others
directly or even devouring some cells whole.
While this sounds brutal, and it is, this process
is highly regulated and doesn’t hurt the mom. It
is another test. The mom’s body monitors carefully
how the embryo is doing – is it growing quickly
or if it is more chill. If an embryo has genetic
damage or chromosomal abnormalities it will spend
way more energy on repairing itself and maybe
grow more erratically. This makes it metabolically
noisy, releasing loads of chemicals the mom’s
immune cells pick up. Which provokes them and
makes it more likely that they will destroy it.
On the other hand, if the embryo is weak it is
metabolically too quiet and the mom will stop
talking to it – which also ends the pregnancy.
This embryo is just right, so the mom’s cells
release a flood of different chemicals to help
it out, support it growing and most importantly,
they activate her immune system. Usually this
would be very bad – the immune system kills
everything that is not part of the mom’s body,
and this embryo is clearly not part of her. But
the immune cells of the uterus surround the embryo
and start helping it, guiding the trophoblast
to grow further. They create a physical and
chemical safe zone that tells dangerous
immune cells like T Cells to stay away.
The embryo has its own motives and doesn't want to
rely on its mothers goodwill alone – so defensive
trophoblasts send out signals that kill her
immune cells if they get too close and could
start attacking it. Hundreds of cells go even
further and leave the embryo behind. They spread
all across the mom’s body, entering organs, even
the brain. We don’t know what all of these cells
are doing but we think that they are probably
telling her immune system that the embryo is not
to be attacked. That it should be left alone or
even protected. These cells may stay inside the
mother for years or even decades. It is likely
that parts of you are still in your mother.
Around 8 weeks after the egg was fertilized,
the transition from the embryo to
fetus begins – the size of an olive,
its organs begin to form and it turns from a blob
into something vaguely human-like. Is there a
clear point where a clump of cells becomes a
human? Not really – it is a fluid transition
and every society and person - be it morally
or legally - marks this moment differently.
Meanwhile construction trophoblasts
are busy building a spongy fingerlike
structure that expands further into
the uterus, a completely new organ:
The placenta – an organ that you once grew inside
your mother, that was ejected after your birth
and died a silent death while everybody
was busy welcoming you into the world.
In our story the placenta is now the new
home of the fetus. An enormous fortress,
protecting it from microbes that could infect and
kill it in its still pretty fragile state. It even
has its own mini immune system, placental
immune cells that gobble up anything that
poses a threat. Other placental cells creep
along the inside of the mom's blood vessels,
stretching them and connecting the fetus
through the umbilical cord on the other
side. With the blood flow secured, it’s
time to load it with as much food as
possible. The placenta releases hormones
that funnel glucose directly to the fetus,
stealing energy from its mom. If the fetus goes
too far and asks too much, this can sometimes
lead to gestational diabetes for the mom during
the pregnancy, starving her body of energy.
The mothers body is trying to support the
new life, but not at the cost of her own
survival. In a sense the mothers genes inside
the fetus still have a stronger allegiance to
her than to the new being – but it’s fathers
genes don’t. They want the fetus to survive
at all costs. So while there is a sort of
fragile peace between both parties, it is
just that – their interests are not perfectly
aligned and both sides have to deal with that.
In the next few months the fetus will
increase its weight over a billion times,
which demands a staggering amount of energy
and cooperation. They still need to work
together to get out of this alive. But if the
fetus gets to this point in the pregnancy,
chances are pretty high that it will
become a baby like you once were,
a proper human, like you are today.
A being with immense potential.
If you are alive today then you went through
an amazingly brutal selection process. From a
little pack of genes traveling through
an incredibly deadly obstacle course,
fighting for your survival, with desperate
words spoken through chemicals and sneaky
actions trying to outsmart the system,
fighting to be alive in this world.
But this is only one side of the story –
you were also chosen – by hormones that
led you to the right place, by the egg
allowing you to merge into one. By your
mom’s body that liked how you spoke to her,
by her cells that protected and cared for
you. Nourished by an incredible amount
of energy that your mother gifted you.
Our biology is a brutal, unforgiving, but
necessary part of the greatest wonder there is:
The creation of new life, of you, of all of us.
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