Placenta previa | Reproductive system physiology | NCLEX-RN | Khan Academy

Placenta previa | Reproductive system physiology | NCLEX-RN | Khan Academy

November 11, 2019 36 By Kody Olson


– [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.