How Pregnancy Is Like Growing an Alien Inside You

How Pregnancy Is Like Growing an Alien Inside You

[ ♪ Intro ♪ ] We usually make pregnancy sound like a wonderful,
9 month period of parental bliss. But if you really think about it, pregnancy
is kind of bonkers. When you’re pregnant, you’re essentially
growing an alien parasite inside of you, having your entire body tweaked and drained to support
something that’s half you, and half some other person. Biologically, it’s quite a feat. And yet, it happens all the time. You, me, and every human on Earth, came from
this strange quasi-parasitic system. So, today, we’re going in utero. We’ll talk about how embryos develop, how
they get their food and get rid of their waste, and do all this while staying under the radar
of their parent’s immune system. At the center of it all is the placenta. It’s an organ — a set of tissues dedicated
to a particular task. And it’s actually the first organ that you
make, and the only temporary organ in the human body. Once a baby pops out, the placenta comes out
as afterbirth. Which, to be honest, looks a little like something
from the movie Alien. Since nobody needs the placenta anymore, it’s
usually thrown away. These days, though, eating it has also become
a trend, even though researchers haven’t found any health benefits. But this weird organ is what’s been making
all 9 months of pregnancy possible! The placenta is made from cells from the fetus
and the pregnant parent. And it basically serves as an interface between
the two. It helps deliver food, dump waste, and exchange
gases like oxygen and carbon dioxide that are critical for life. The placenta begins to form within a week
or so of fertilization. In case you need a recap, this is when the
sperm meets an egg to form a zygote, or fertilized egg. The zygote divides a few times as it travels
to the uterus, or womb, and becomes a semi-hollow ball of cells called a blastocyst. The blastocyst is important because it’s
made of two types of cells. There’s a clump of cells inside, which will
go on to make the embryo and eventually the fetus. And there’s a single layer of cells on the
outside, known as trophoblasts, which will form the fetal part of the placenta. These outer cells invade the lining of the
uterus to make the whole thing stick, or implant, and truly kick off pregnancy. Different types of trophoblasts then go on
to form the amniotic sac, which is really just a protective bag of fluid the embryo
floats in. It’s thin, but super tough, and provides
cushioning and room to grow. Inside the amniotic sac, the embryo gets to
work quickly, starting to develop its nervous system, and then its heart and blood vessels,
followed by other organs. Driving all this growth is food, thanks to
what’s essentially a big pool of parental blood. Yep, it turns out that embryos are kind of
like vampires. See, early on, the invading trophoblasts destroy
a bunch of the tissue in the uterine wall and remodel the blood vessels. Trophoblasts attack vessels from the inside
and outside, turning them into limp, open pockets of blood. Those pockets help make sure that there’s
a steady flow of parental blood rich in oxygen and nutrients like glucose, amino acids, and
fatty acids — all of which the embryo needs to grow. Those chemicals get passed on to the embryo
through a branching network of blood vessels that travel through the placenta and feed
into the umbilical cord. But there’s always a membrane between parental
blood and fetal blood. The blood pools are close enough for nutrients
and gases to get passed back and forth through the membrane — usually through diffusion,
which is when molecules move from higher concentration to lower concentration. But they’re still separate. Because direct contact could cause problems…
like total annihilation of the embryo. We’ll get to more of this later, but because
a baby is half another person, its cells are full of proteins that the pregnant parent
doesn’t recognize as their own. Direct contact would mean the immune cells
in the parent’s blood would mount an attack to get rid of the half-foreign being — like
it would stomp out a flu virus. To make sure that doesn’t happen, we evolved
a placental system that always puts a membrane in between the blood supplies. So, through processes like diffusion, a parent
delivers sugar and other goodies to the baby. And the baby can dump all of its carbon dioxide
and waste products into the parent’s blood pool. Garbage problem solved! Based on all of this, you can probably tell
that pregnancy is kind of blood-intensive. And one of the biggest changes to someone’s
body, besides making space for a new human, is to their cardiovascular system. Blood vessels across their body widen. Their heart rate ticks up an extra 10 to 20
beats per minute. And ultimately, they pump 40 to 50% more blood. You can actually see some of this extra blood
flow as a rosy blush on people’s faces, which is sometimes called pregnancy glow. It happens partially because of the increased
blood output. Beyond glowing skin, though, these changes
matter to the health of both humans. They make sure the placenta gets enough blood
to constantly refresh the blood pool — something that happens 2 to 3 times per minute. That way, the baby gets enough oxygen and
nutrients. And the additional blood flow also helps the
parents’ kidneys process the extra waste that’s getting dumped into their bodies. They’re eating and excreting for two. One of the ways scientists think this happens
is through a hormone called relaxin, which dilates blood vessels to make them wider. This allows pregnant people to safely handle
more blood pumping through their bodies, instead of boosting their blood pressure. Relaxin levels naturally go up in people during
ovulation, and stay high if they become pregnant. The hormone might also help later in pregnancy
by relaxing ligaments in the pelvis to make it easier to deliver the baby. Other hormones, like estrogen, might also
be involved. And while the parent’s body initially makes
most of the hormones, eventually a bunch are made by cells in the placenta. So basically, the fetus ends up controlling
a lot of the show. And some of this stuff borders on mind control. Progesterone, for instance, is a hormone that
keeps the pregnancy going, and even changes how the parent breathes. This hormone tells the brain to lower the
amount of carbon dioxide in the body, so pregnant people will actually take bigger breaths. This provides more oxygen, which the baby
needs, and also makes it easier to get rid of the carbon dioxide, so breathing is more
efficient. Clearly, the fetus has no qualms about doing
whatever it needs to do to get things for itself. And by the eighth month, some researchers
think that about 25% of the proteins a fetus gets are used to make hormones to manipulate
their parent. Now, if the cardiovascular stuff sounded intense,
think about the fetus’s near-monopoly on glucose. Not only does the baby want more blood, it
wants more sugar in that blood. And one way the fetus accomplishes that is
to make their parent diabetic. Or at least, diabetic-like. The fetal part of the placenta releases a
hormone called human placental lactogen, among other chemicals, that decreases the pregnant
parent’s insulin sensitivity. Insulin is a hormone that controls how much
glucose is in your blood. When it binds to cells, it tells them to stop
releasing sugar into the bloodstream, and to start taking up more sugar from the blood. So when you’re less sensitive to insulin,
you don’t clear your blood of glucose as quickly, so you have higher blood sugar. This can work to the baby’s advantage, which
is probably why it happens. The fetus can take all of that extra sugar
for itself. Now, this whole fetal exploitation of their
parent is usually okay. But sometimes, especially in people who already
have diabetes or are at higher risk, this can lead to something called gestational diabetes. In gestational diabetes, the parent’s blood
sugar goes too high, and the fetus ends up getting way more sugar than it needs. If that happens, the baby can grow really
big — to the point where it’s not safe to do a natural delivery, and doctors do a
C-section, which involves surgery. There can also be other complications, for
both the parent and the baby, so doctors tend to keep close tabs on sugar levels. The fetus’s biggest trick of all, though,
might be its ability to stay out of the way of the parent’s immune system. After all, the baby is still a guest… and
kind of an interloper. Yes, it’s walled off in a fluid-filled sac,
and protected by the placenta. And these physical barriers go a long way
in explaining how pregnancy can even work at all. But the fact remains that many cells of the
placenta come from the fetus, so they’re chock full of a mix of proteins — some foreign
to the pregnant parent and some not. But, somehow, they touch parental tissue with
little to no problem. Immunologists have wondered about this for
more than 60 years. At first, they assumed that pregnant people
simply didn’t make immune cells that recognize the fetus. But we now know they do. So, how do developing fetuses avoid damage? We don’t know the full story, but one big
way seems to be a surge in a type of immune cell called a T regulatory cell. These cells basically dial down immune responses,
rather than increasing them. And they show up in large numbers as soon
as the blastocyst implants in the uterus, and possibly even earlier. Another clue comes from looking at the proteins
on the surface of the trophoblasts — the placental cells that do the invading. Normally, cells have proteins on their surfaces
that different immune responders can recognize. That’s how an immune response starts, and
your body can tell you’re infected with a bunch of bad bacteria. But trophoblasts are missing a lot of these
surface proteins, or have slight changes to them. And that could give them a kind of invisibility
cloak. The immune cells in the uterus are also different. Normally, natural killer cells do exactly
what their name implies — they kill. But the ones in the uterine lining don’t. Basically, they still have the proteins they
need to kill, but the cells also have inhibitory receptors. And when those inhibitory receptors are activated,
it prevents the release of those deadly proteins. These natural killer cells are around when
the trophoblasts are reworking the parent’s blood vessels, and might even help by pumping
out special factors. In mice that are engineered to lack natural
killer cells in the uterus, the parental part of the placenta doesn’t grow properly, and
baby mice are born abnormally small. There are other ways that researchers think
babies avoid detection, too. And most of the time, they pull it off. But scientists are now realizing that many
fertility problems might actually be immune problems that crop up very early in pregnancy. If an embryo can stick around long enough,
though, they might get the last laugh. Because decades after pregnancy, scientists
have found fetal cells still hanging out. This is called microchimerism, and may be
one of the strangest features of pregnancy. It’s actually a two-way street: kids end
up with cells from their parent in them, too. Babies can even end up with cells from their
older siblings or older generations, kinda like a Russian nesting doll of past pregnancies! But people finish pregnancy with far more
cells from their children than the other way around. Doctors haven’t figured out if they’re
meaningful in any way. Some have proposed that these foreign cells
can cause autoimmune diseases later, although that hasn’t been fully demonstrated yet. If you’re feeling sentimental, though, you
can think of them as a literal keepsake in your body. Pregnancy means you’ll always carry part
of your kid with you, whether you want to or not. Thanks for watching this episode of SciShow! We wouldn’t be able to make all these videos
about how weird humans are without our on Patreon. patrons So if you want to help support this show,
you can go to And don’t forget to go to
and subscribe! [ ♪ Outro ♪ ]

100 thoughts on “How Pregnancy Is Like Growing an Alien Inside You

  1. Okay, like I got ROASTED by an entire class when I suggested this back in High School and College, but here is someone making money on YT content from exactly the same things I said!!!

  2. God it was so annoying to hear you constantly say pregnant people. It's pregnant women goddamnit. A man can't get pregnant. It even SOUNDS forced when you are saying it.

  3. A science channel of all things should feel comfortable enough to not try so hard to be p.c. Facts are facts. It's okay to use the term "female"…

  4. It is so funny to see many snowflakes got triggered that in this video the speaker used pregnant people rather than pregnant women. LOL I even didn't notice that after I finished watching this video. Is it wrong to say so? Logically, there is nothing wrong. Some people get pregnant, some don't. Unless you isolate women from people, implying women are not people, then you will insist that the speaker should use pregnant women. The speaker did not say male can get pregnant. So any accusation of this line can stop. You are putting your words into other people's mouths.

  5. there are some assholes in the comments (which honestly shouldn't surprise me) but as a trans person who hopes to someday have a baby, your careful use of language meant a lot. thank you.

  6. Commentator dressed like a boy about to go skiing, not a woman talking about the most womanly thing that we are honored to do: to give birth

  7. Omg that diabeties test was gross they gave me an orange flavored drink and it was so sweet it burned my throat 😖😖😖

  8. My mom had gestational diabetes with me..Not with my brothers though.
    Maybe that's part of the reason why I'm trans? Maybe you guys could do a video on trans* (transgender/transsexual)

  9. Comparing the propagation of the species to growing an alien just seems wrong, especially since the concept of aliens is fictional….it's a miraculous phenomenon..plain and simple..

  10. Actually alien pregnancy, in movies, is meant to mimic real life pregnancy to produce a uncanny valley effect, and also to scare the crap out of mothers, and whoever else is scared of pregnancy.

  11. Technically eating placenta is cannibalism, but if you're going to be a cannibal you might as well eat yourself. I don't know, it just seems very weird to me.

  12. I'm confused. Why are people who find the idea of comparing pregnancy with parasitism even watching this video? IT WAS RIGHT IN THE TITLE. Also, speaking as someone who has gone through the process, finding the experience of pregnancy strange doesn't stop you from also finding it fascinating or from loving your child. Your mind has the space for all of it.

  13. as a male I can guarantee that my sole purpose was to create a child… all other things is undecided in my life but I'm experimenting before creating a child and I will experiment after sowing my seed…

    Yes, I may only have 1 purpose in evolution but I seem to feel I need more than that in life… hears yelling in the background RAISE YOUR CHILD!

  14. interesting… it might be good to study if we can change the DNA of women if we implant them with altered DNA embryo's that have higher immune system such as one that is immune to plague and AIDS and see if the female adopts that into her DNA… I know it sounds cruel but in reality…. if a cure can be as simple as getting pregnant then who knows… maybe, just maybe people would be more inclined to take one for the team and try it out?

  15. Just say woman or mother. Jeeze. Dont understand why people are trying to say men can be pregnant when they can't. It's not in their genetic make up.

  16. I appreciate how they said “they” and “people” instead of “she” and “women/female”. Because trans men, non-binary people etc. who have vaginas exist. And acknowledgement and representation is really important!

  17. Ok SciShow, I really enjoy watching your videos, and I understand political correctness & not wanting to offend any of the "snowflakes" out there, and there's far too many of them, but I don't think its all that necessary & really is not important enough to be used on a Science & Biology channel!!
    So with that being said, why the hell are your videos saying things like pregnant parent and person?!? I mean come on, enough already! The first few times I've heard pregnant parent/person I thought hey that's kind of strange, why don't they just say female or woman or mother or expecting woman/female? (Or any other combination of words indicating that the human being carrying the fetus or embryo ect is a female woman)
    It's just gotten to be so annoying since this whole video is on the topic of the human reproduction process!
    So if anyone from this channel sees this comment, please we have not gotten to the point where any other humans except for us females/women are carrying these aliens around inside of our bodies!
    (And make sure you call the humans that produce the sperm fathers & males too! Don't want to exclude them either!)

  18. And BTW I posted my previous comment before reading the comments from other viewers, and from what I am not alone in my annoyance with the SciShow people using "pregnant parent/person" and i am glad too!!
    I totally understand there's humans on this planet that are "gender neutral" & "non-binary" & whatever other words there are to describe the people who do not want to use the normal terms like male/female & mother/father.
    And I know there are transgendered folks who switch to the opposite from what Mother Nature gave them at birth, and as embarrassing as this is to admit to, I have even watched a show with the term Pregnant Man in the title, it was from either TLC or Discovery Channel, I cannot recall since it was several years ago that it aired & I watched it. So the Only reason I watched it was for the totally misleading title!! I thought Science found a way to make a male carry a child. NOPE! It was just about a transgendered person who was born a female & switched over to male through surgery & hormones.
    So my point is that since there is no way for a male to carry & have a baby, (even though a person born a female can switch over to being male, it's not exactly such, since there's no way to transplant the entire reproductive system of a male or a female to the other gender)
    SO until there is a way to give the opposite reproductive organs/systems to a person of the opposite gender, can we please just call a person carrying & having a baby a mother/female/woman? And the person who has all the male reproductive organs/systems the father/male/man??
    It's nothing against the transgendered folks at all, I don't care what you want to be or what you want to call yourself personally, unless however you do not disclose the fact that you are a transgendered man while hitting on me in a bar or the library or wherever and I think that you were born a man, since that is really unfair & misleading!
    Let's just call a person with a uterus a woman/female/mother and a person carrying & depositing the sperm a man/male/father!

  19. It’s now about 25 years ago that I gave birth to my daughter. Now that she is a grown woman, I find it hard to let her go. Finding out that there will always be a part of her that stays with me makes me unbelievably happy. I’m so happy that I watched this video.😃

  20. People think I will change my mind about wanting a baby…but I'm 21 and still have no frigging interest in a baby, or pregnancy.

  21. Mom I want the cells you stole from me
    In return I’ll give your cells back
    Your favourite parasite

  22. You are all insane. You’re calling babies parasites. How about looking at yourselves and seeing how parasitic your own behaviour is to everyone else?

  23. whatever it is useless women do not pass the immune support or antybodies for immune support they pass disease and damage the fetous

  24. Why keep saying parent when it is only WOMEN who get pregnant? It’s a mother not a generic parent. Talk about PC nonsense gone wild 🙄

  25. Pregnancy is slightly horrifying, but that may be because I as an individual am not equipped to mentally handle the inner workings of the human body.

  26. I'm sorry, but the blanket substitution of "parent" for 'mother' grated on my nerves. If men ever start getting pregnant & bearing children, that substitution might be politically correct and necessary, but I doubt that'll ever happen.

  27. The weirdest thing about possibly having cells from your siblings is I have an older half sister. If I had any of her cells. That means I could have cells that originated from someone I'm not even related to which is super weird

  28. I would just like to appreciate the fact that you guys have committed to using gender-neutral language, without any announcement, ceremony, or attention. It shows just how easy it is to do. Thanks y'all ❤

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