[00:00] 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.
[00:12] 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.
[00:25] Let us zoom in and observe this brutal  struggle for survival – that in the   best case ends in one of the most magical things: Our story begins with an army of tens of millions  of sperm cells that immediately face death.
[00:41] They are on a tight timer before  they run out of energy and die. Like all bodily openings, the female reproductive  tract is a fortress defending against invaders.
[00:54] 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  
[01:09] The acidity kills millions, especially of  the weaker sperm within the first half hour,   Seminal fluid is alkaline and so it makes  this first obstacle a little less deadly.
[01:23] 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  
[01:37] 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  
[01:54] 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.
[02:08] 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  
[02:20] 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.
[02:35] The egg stops being part of its  mother – it is something else now. Not a human yet, but the potential to become one.
[02:47] But its mom’s body hasn’t decided  yet if she wants to say it out loud. For a few days it rapidly divides and grows to a  couple of hundred cells, while traveling down to  
[03:01] the uterus, the part of the female reproductive  system where it will try to make its new home. 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:
[03:18] 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.
[03:31] 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. the young embryo moves along the  uterus and tries to take a hold.
[03:49] 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.
[04:03] 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.
[04:16] living beings no longer completely align anymore. body will eject the embryo if  it doesn’t see it as viable.
[04:28] death situation and tries  to stay alive at any price. So it’s not asking politely but deploying  thousands of infiltration units:
[04:40] Bubbles filled with genetic  material, kind of like a human virus! so they help it attach itself,  rather than rejecting it. During this time one of the weirdest  features of pregnancy occurs:
[04:55] 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.
[05:12] 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  
[05:27] 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.
[05:42] While this sounds brutal, and it is, this process  is highly regulated and doesn’t hurt the mom. The mom’s body monitors carefully how the   embryo is doing – is it growing  quickly or if it is more chill.
[05:55] 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.
[06:09] 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.
[06:22] 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, activate her immune system. Usually this would be very bad –  the immune system kills everything  
[06:36] 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,   They create a physical and chemical safe zone that  
[06:49] 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 – kill her immune cells if they get too  close and could start attacking it.
[07:04] Hundreds of cells go even further  and leave the embryo behind. They spread all across the mom’s  body, entering organs, even the brain. probably telling her immune system  that the embryo is not to be attacked.
[07:19] 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 –
[07:36] and it turns from a blob into  something vaguely human-like. Not really. person – be it morally or legally  – marks this moment differently.
[07:55] 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
[08:09] 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.
[08:24] placental immune cells that gobble  up anything that poses a threat. stretching them and connecting the fetus  through the umbilical cord on the other side.
[08:38] 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, If the fetus goes too far and asks  too much, this can sometimes lead
[08:52] 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  
[09:08] to her than to the new being –  but it’s fathers genes don’t. 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.
[09:24] In the next few months the fetus will  increase its weight over a billion times,   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  
[09:39] that it will become a baby like you once  were, a proper human, like you are today. If you are alive today then you went through  an amazingly brutal selection process.
[09:52] 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.
[10:07] 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 her cells that protected and cared for you.
[10:23] 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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