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Pregnancy is quite a feat. The embryo manages to develop, get food, and get rid of their waste, all while staying under the radar of their parent’s immune system.

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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

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