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Placenta previa

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Nauroz Syed.

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Video transcript

- [Voiceover] So we all have that friend, that one friend who is never in the right place at the right time. Well it turns out that placentas also have that one friend. So what do we mean by that? Well placentas are normally located in the upper part of the uterus. So near this portion of the uterus called the fundus, the fundus. And there's this condition called placenta previa, placenta previa, in which the placenta is abnormally positioned. So instead of being found in the upper part of the uterus, it's found somewhere in the lower part of the uterus, kind of like this, near the portion of the uterus called the cervix. So instead of being found in the fundus, it's found near the cervix. And the placenta can either partially or entirely cover up the internal opening to the cervix, so this portion right here called the internal os or the internal opening of the cervix. So why does this happen? Well it turns out that the placenta will form wherever the embryo implants. So if instead of implanting near the top of the uterus like it's suppose to, the embryo implants in the lower part of the uterus, the placenta will also form in the lower part of the uterus, leading to placenta previa. So if you look at this diagram, you might ask yourself, well yeah sure the placenta is located in the wrong place, but is that necessarily a bad thing? And the answer to that is sometimes. So early on in the course of a pregnancy, if a woman is diagnosed with placenta previa, it tends to not be that big of a deal, and the reason for that is because as the pregnancy progresses the baby grows and as the baby grows the uterus gets larger and it stretches out. And with the stretching out of the uterus, the placenta gets dragged along to a higher position. So if a woman is found to have placenta previa early on the course of her pregnancy, as the uterus grows, the condition tends to self-resolve, and the placenta tends to find itself in a normal position by the end of the pregnancy, so much so that it's pretty rare to still have the condition by the time the baby is ready to be delivered. So for that reason, if a woman is found to have placenta previa early on in her pregnancy, it tends to not be that big of a deal. If however, she's found to have placenta previa later on the course of a pregnancy or if she was found to have it earlier on and it still persists by the time that she's ready to be due, it tends to be a very big deal. And the reason for that is that in this location the placenta is really prone to bleeding and I want to show you why that is. So I kind of want to expand this or zoom into this part of the uterus. So we already talked about the structure of the placenta in another video that's called, Meet the Placenta. So over here we're just briefly going to gloss over it. So this is the uterus or rather it's the wall of the uterus and the uterus is a muscle, right? It's a muscular organ. So in a non-pregnant woman there are all these blood vessels or arteries, they're called uterine arteries that come down to the uterus and they supply it with oxygenated blood. Because again, the uterus is a muscle so it needs lots of oxygen. So what happens in pregnancy is that these uterine arteries become really juicy and plump and they actually cross through the wall of the uterus and they squirt out all this blood. Kind of like we said the jets in a hot tub do. And they squirt out all the blood into the placenta. So the placenta is kind of like a pool of blood and the baby also sends down blood vessels into the placenta so that it can get access to all the oxygen and the nutrients from the pool of blood that is the placenta. So something pretty amazing happens late in the course of the pregnancy when the baby is pretty close to being delivered. So as you might know, if you look at this diagram over here. So as you might know, when babies make their exit from the uterus, they do so head first, and their heads tend to be pretty big, and you can see that this thick and plump part of the uterus, the cervix, is kind of standing in the way of the baby's head. So what happens towards the end of the pregnancy is that the wall of the uterus over here, at the cervix, thins out to widen the passage for the baby. Now normally this is a good thing. It's an awesome thing, but if the placenta is sitting right here like it does in placenta previa, it can be a pretty dangerous thing. So when the wall of the uterus thins out, the attachment here, the attachment between the uterus and the placenta is actually strained. It starts to become weakened and the placenta starts to detach. And as you can imagine these uterine arteries, which are actually in the wall of the uterus are being tugged on in this direction as the wall thins out, but since they're also attached to the placenta, they're also being tugged on in the opposite direction. So pretty easily the tension on these arteries can cause them to rupture and blood leaks through into the vagina. That's actually how placenta previa tends to present with vaginal bleeding late in the course of the pregnancy so that's called antepartum bleeding, so antepartum vaginal bleeding. And antepartum refers to the portion of the pregnancy that's after the 20 week point. So it turns out that there are two major causes of antepartum bleeding. One of them is called placental abruption, which is something that I'm going to talk about in it's own dedicated, committed video, and the second cause is placenta previa. And there's a really important way to tell them apart. So placental abruption tends to present with painful bleeding where placenta previa tends to present with bleeding that's not so painful, so painless vaginal bleeding. So if a woman presents with painless antepartum bleeding, so painless vaginal bleeding after 20 weeks into her pregnancy, she's suspected of having placenta previa. And if that's suspected, if placenta previa is suspected, the very first thing we do is an ultrasound, so we slap an ultrasound onto the belly to visualize the location of the placenta. And if the woman is found to have placenta previa, it is really important. We make a pretty big point not to do a pelvic exam, so not to insert either a speculum or a finger through the vagina, and that's because either of these two maneuvers, inserting either a speculum or a finger through the vagina can cause or it can worsen the vaginal bleeding. Okay, so if a woman has placenta previa, what's next? Well a lot of that depends on what she looks like when she presents. So if she comes in with pretty minimal vaginal bleeding and she's hemodynamically stable, so if her blood pressure and her pulse are okay, then she's probably going to be sent home on bed rest and she's going to be told to avoid strenuous physical activity and to avoid sexual intercourse because that in of itself can cause bleeding. And then she'll probably be brought back to the hospital around 34 weeks into her pregnancy to have a C-section and the reason why we choose to do a C-section rather than proceed with a vaginal delivery is pretty apparent in this diagram. So remember when we said that the baby passes through head first, so you can see that the placenta, in placenta previa, is blocking the baby's path, and during delivery the strength of the uterus contracting and pushing on the baby through can shear the placenta, which remember is a pool of blood pretty much, so that can cause massive hemorrhage for the mom, so that's why we offer a C-section to avoid this potentially fatal situation for the mom. Now if instead the mom presents with massive bleeding and she's hemodynamically unstable, so if her blood pressure and her pulse are not doing so okay, then we go in for a C-section at that time regardless of how far along her pregnancy is or how old the baby is to prevent any consequences for the mom. So I guess that leaves us with a pretty important question and the question that a lot of people tend to be interested in and that is what increases your chances of having this condition? And one of the biggest risk factors I guess you can say is having a history of placenta previa. So if you had placenta previa with a previous pregnancy, you're much more likely to have it with any future pregnancies. Another pretty big risk factor is multiple gestations, so women who have twins or triplets or quadruplets or more babies, because each of these babies has its own placenta and you can imagine that the higher number of placentas, the greater the chances of having one fall into this area. Alright, so it's simply a probability game. And there are a few other risk factors like having surgery on the uterus or having a uterine scar that'll increase your chances of having placenta previa. So there you have it, placenta previa or the placenta that's never in the right place at the right time.