Monochorionic Diamniotic Twin Pregnancies - HD

Introduction

I am Dr. Tara Morgan from San Francisco, California, and today I'll be speaking on monochorionic twin pregnancies, making the important findings.

Types of Twin Pregnancies

Most twin pregnancies are dizygotic, meaning they arise from two separate eggs. So they form separately and have two chorion and two amnion. So they're all dichorionic and diamniotic. Importantly, they also all have separate placentas and sacs.

30% of pregnancies are monozygotic, meaning they come from a single egg, which at some point divides. These pregnancies depending on when the egg divides can be dichorionic, diamniotic, monochorionic diamniotic, which is what we're gonna focus on today, or monochorionic monoamniotic. And importantly, monochorionic pregnancies share a placenta.

So it's really important to determine the chorionicity because it affects the management of twins, and it's different for monochorionic twins and it is for dichorionic twins.

Monitoring and Risks of Monochorionic Twin Pregnancies

So firstly, we monitor monochorionic twin pregnancies every two weeks instead of every four weeks like we do, starting at 16 weeks for dichorionic pregnancies and monochorionic pregnancies overall have a higher rate of mortality and morbidity. They have a higher incidence of anomalies and likelihood of IUGR, and they particularly have unique monochorionic syndromes, which is really gonna be the focus of today's talk.

So in order to take care of these patients appropriately, the best thing you can do is first accurately determine the chorionicity. And this is something that you need to take some time in learning how to do.

Determining Chorionicity

So first of all, believe what you see over the history, even in this day and age where we see a lot of fertility assistance, such as in vitro fertilization, you could still have a single egg split into two and become monochorionic. So don't let the history completely dissuade you. It's most reliable to determine the chorionicity in the first trimester. So you wanna do that as early as possible.

And the findings you wanna use throughout pregnancy to help determine chorionicity are membrane thickness, number of placental masses and fetal gender.

So this is a first trimester ultrasound at about six weeks. And you can see on the left hand image, there are two echogenic rings. So two gestational sacs that have formed. We see the anechoic fluid in the center, and there are two embryos that you can see sliding by on the cine clip. It's too early really even to see the amnion here, but this doesn't matter because we see these two echogenic rings. This is chorionic tissue, and this is definitively a dichorionic pregnancy.

On the contrary, this is a pregnancy also at about six weeks where we see a single echogenic rim of chorionic tissue. So this is a monochorionic pregnancy. Also on the cine clip, you can see there's two embryos as the movie clip slides by, and you can see the amnion in this case, they look sort of like a soap bubble thin membrane surrounding each embryo.

The helpful thing here about early in the first trimester, they have not yet fused, but sometimes it's actually hard to see the amnion. So what you can do is then count the number of yolk sacs. The number of yolk sacs is going to equal the number of amnion. So this is a monochorionic diamniotic pregnancy.

Another finding that's really helpful is if you see this tissue in between the membrane of the so-called twin peak sign. So this is a dichorionic pregnancy because placental tissue only grows along the chorion. So these two twins have, both have their own placenta, they're along the anterior wall of the uterus, but they fuse together. So you can't really separate them visually. But because of this twin peak sign, we know that there's both chorion and amnion opposed in the membrane. And therefore this can be definitively called a dichorionic pregnancy.

Remember that the amnion is a specular reflector, so that means that it's flat and linear and it catches the ultrasound beam when it's perfectly in the perpendicular plane. So we can use this to our advantage as a way to see the amnion. But you also don't wanna mistake this for being a thick membrane because of the really bright reflection that you can see when you're centered over the amnion.

So as pregnancy continues, the membrane thickness changes. It gets thinner in both types of pregnancies. So it's really important to establish the chorionicity as early as possible.

You can see here in the second trimester on the left, we have a dichorionic pregnancy, and on the right a monochorionic pregnancy. So both membranes are relatively thin compared to earlier in the gestation. However, the membrane on the left looks a little bit thicker. It's composed of two layers of amnion and chorion. And on the right we just have two layers of amnion. So you can decide that you think that the one on the left is dichorionic pregnancy. But it's difficult to be completely certain. So beware of these thinning membranes.

Here we see a pregnancy at 13 weeks where we have the twin peak sign. So we know this is a dichorionic pregnancy, but look what happens when we get to 28 weeks. The membrane there is much thinner. It looks a little bit thick, but it's much thinner than it was earlier on. And if you only had this image, you could potentially be confused about the chorionicity.

You never wanna change the diagnosis of the type of chorionicity from the earlier image based on the appearance of the membrane when it becomes thinner.

If you have two placental masses, it's really helpful. You can definitively say that it's a dichorionic pregnancy because you know you have two separate placentas. What happens a lot of times is that if there are two placentas, they fuse or merge together, as we've seen earlier in the twin peak example. So it can be hard to tell whether or not there are in fact two placentas. So if you see one placental mass, you can either have a dichorionic or monochorionic pregnancy.

Similarly, a difference in gender is really helpful. So you're definitely knowing that this pregnancy originated from two different eggs. And so this has to be a dichorionic pregnancy. However, if the genders are the same, it could be from a dichorionic or a monochorionic pregnancy. So it's actually 65% of twins that have the same gender. We have the 50 50 likelihood of all of the dichorionic pregnancies. And then because all monochorionic pregnancies are gonna be the same gender, they add a higher percentage to the total of pregnancies that are the same. So we can determine chorionicity.

Just to summarize by looking at the membrane, if it's thick, you know you have a dichorionic pregnancy, so you're done. If it's thin, you can go on to look at the gender. If they're different, then it's a dichorionic pregnancy. If they're the same, you're still in that situation where it could be either. You can then look at the number of placental masses. If they're two, it's dichorionic, and if there's only one, then it still could be a monochorionic or dichorionic.

So if you have a thin membrane, same gender, and one placental mass, and this is usually occurring in the second or third trimester, you could be stuck still not knowing the chorionicity.

Significance of Monochorionic Twin Pregnancies

So now that we've looked at how to determine chorionicity and talked about how important it is, we can move on to why it's significant to have a monochorionic twin pregnancy or one placenta.

So these pregnancies all share vascular anastomoses and there is usually some degree of unequal sharing of the placental mass between the twins. These two factors can lead to differences in size or weight between the twins and also fluid discordance. And really what we're seeing happening here is an overlap of these processes.

Vascular Anastomoses in Monochorionic Pregnancies

So a normal placental vascular connection that we see on the left between the arteries and the veins. You see that the artery is entering the placenta and then exiting back through the vein to return to the fetus. And if you look at the placental surface, you'll notice that these two vessels are parallel in their position running together.

In a monochorionic twin pregnancy where we have two umbilical cord insertions, what you can get is an entering artery that then exits and goes at the opposite direction to the opposite twin. So this in fact acts sort of like a shunt where one twin is sending blood to the other twin. Again, looking at a schematic of the whole placenta, you can see that these vascular connections exist. So we have arterio to venous and venoarterial in either direction, so these can potentially be balanced or unbalanced to different degrees.

And then importantly, there's another type of anastomosis we'll talk about called the arterio to arterio anastomosis. This is two arteries kind of going up against each other from both twins. So the blood flow in fact doesn't really move too much in either direction. And this is thought to have a protective effect against some of these other unbalanced connections. And when we see this, it just helps to predict a better prognosis for the pregnancy.

Looking at the placental surface, we'll look at the same thing and identify some of these vascular connections. So you see on the left here, we've got a normal arterio to venous connection. So we're seeing an artery, a vein exit and enter the placenta at the same place and look how they're running parallel back to the same cord insertion.

Here is an abnormal arterio to venous anastomosis. So you can see that we've got a vessel coming from one umbilical cord entering the placenta and then exiting to the other umbilical cord. And then just to show you, this is the arterio to arterial anastomosis which runs along the surface.

Monochorionic Twin Syndromes

So the monochorionic syndromes that we're gonna talk about are unequal placental sharing, twin to twin transfusion syndrome or polyhydramnios of the recipient like twin and TRAP or twin reverse arterial perfusion syndrome, also known as acardiac twin. And again, just to emphasize that these do tend to overlap.

You can accurately make these diagnoses or avoid missing these diagnoses by identifying a few certain anatomical findings. So you wanna first identify the cord insertions for both twins. You wanna assess the fluid, which is done by the deepest vertical pocket for both twins. Look for the presence or absence of a fluid-filled bladder. And in relevant cases, look whether or not the doppler is abnormal or normal.

Unequal Placental Sharing

So if you look at this large field of view image from a twin pregnancy, before I put any kind of measurements on this or give you more information, you can see that the twin on the left is larger than the twin on the right. So we have one at the 51st percentile and at the 16th percentile. So birth discordance is common in twins and it's considered significant if it's equal to or greater than 20% discordance. So this is not just seen in twin to twin transfusion syndrome. It's seen in a large percentage of these pregnancies without the presence of twin to twin.

And importantly, in one study from 2010, they showed that a large portion of these have a velamentous cord insertion. Other types of cord insertion other than central such as marginal or eccentric towards the edge didn't seem to matter with developing weight discordance. And if you looked at the placenta of these pregnancies, there was unequal sharing of the placenta more than half of the time. So this is something that really is just seen pathologically and we can't do that during the ultrasound, but we can kind of infer or make some conclusions about how much of the placenta is going to each twin by the position of the cord insertion.

So just showing this in the schematic, you can see how these cord insertions can potentially affect growth, affect treatment, and explain unequal sharing. So if you have a velamentous cord insertion, it's actually inserting along the uterine wall and then the vessels have to travel to get to the edge or a portion of the placenta. Whereas if you have a central insertion or even more of an eccentric insertion, you're really getting these blood vessels in more of the meat of the placenta. So you can see how in these circumstances you would potentially have less placental mass for the twin with the velamentous cord.

Assessing cord insertions can be a little bit challenging. So it's worth taking some time to look at how to most accurately do this. So here is a central cord insertion. Really I call a central cord insertion anywhere that's not right at the placental edge. So we can see this is a posterior placenta and this is the cord insertion here. It's got sort of a stellate appearance. If you see these vessels going in this radial fashion into the placenta, you can be confident that you're in the right location.

Sometimes, especially as the fetus gets larger and there's less relative amniotic fluid, you could mistake a portion of the cord lying along the placental surface for the potential insertion. And this will help to avoid that. It's also really important to look at the cord insertion in two planes, and that's because sometimes you can see what looks like a central cord insertion and then you turn in the other plane and you're actually towards one of the edges. So here we're seeing that this cord insertion is central on both the trans and longitudinal planes.

On the contrary, this is a velamentous cord insertion. So on the left we see the placenta and the arrow shows the edge of the placenta and the cord insertion is just off to the right in the uterine wall. So this is considered a velamentous cord insertion. If you look at this video clip, you can see a part of this echogenic placental tissue. And then as we scroll off of it, then you can see that the cord insertion is actually coming in and inserting on the more hypo area, which is the uterine wall.

A marginal cord I call when you have the placental cord insertion going right into the edge of the placenta. So you really don't see a difference between the placental edge and where the cord is inserting. And then eccentric is really everything in between. So this cord insertion is not quite central, but it's not at the placental edge. And this is a term that I don't actually use very often because it really doesn't end up mattering. Really anything that's central or even close to the middle of the placenta tends to have similar outcome.

So talking about unequal placental sharing, which I think is more of a weight problem between the fetuses where the size discrepancy is equal to or greater than 20%. Often we do see a velamentous cord insertion in the smaller twin. So one of the reasons to identify where the cord insertions are, you can have low or normal fluid for the smaller twin, but you're not seeing this polyhydramnios oligohydramnios like I'm gonna show you in examples of twin to twin transfusion syndrome. You expect to see a normal bladder and stomach for both twins and the doppler is usually normal. I put a star there because there are some exceptions. And what you really wanna look for and hope to find to kind of confirm this diagnosis is the presence of an arterial to arterial anastomosis.

These pregnancies still need monitoring for growth and for the potential development to twin to twin transfusion syndrome.

So this is an example of unequal sharing. We've got normal bladders seen by the arrows, both A and B, and we have normal fluid. So between two and eight centimeters for both twins. A and B, this is an arterial to arterial anastomosis. As I said, it's thought to be protective or a good prognostic factor in monochorionic pregnancies and sort of helps with these unbalanced vascular connections.

So if you search for this particular vessel, you wanna look along the surface of the placenta and it has this characteristic red, blue, red, blue. And this is from the back and forth of the arterial flow. And this is why it has this very characteristic waveform. So you can see here it's arterial flow going above and below the baseline. This is one of these things where when you find it, you can be confident that you found the correct thing. Nothing else really looks like this, but it can be a little bit difficult when you're looking for an arterial to arterial anastomosis. And then you don't know, am I just not finding it or is it something that's not actually there. So with a little practice and with looking for that particular characteristic flashing vessel along the surface, this can be found.

This is a normal umbilical artery doppler. And I show you this because I want to contrast it on the next slide. So notice how it's a brisk upstroke and then we've got the presence of diastolic flow throughout the cardiac cycle.

Sometimes when you have an arterial to arterial anastomosis present, it can kind of pull blood away from one of the umbilical cords. So you see here that there's a lack of diastolic flow in this umbilical artery. If this is an isolated finding, it can be within normal limits and you have to put it into context. You don't want to automatically assume that this is an abnormal doppler.

Importantly, it's only about 5% of monochorionic pregnancies that have an AA or an arterio to arterial anastomosis that go on to develop twin to twin transfusion syndrome. And this is why it's so important to look for this finding.

Twin to Twin Transfusion Syndrome (TTTS)

So now to talk about the twin to twin transfusion syndrome, which is an area where we really are able to intervene and positively affect the outcome in monochorionic twins. So as the name, it's a transfusion of blood from one twin into the other, from these vascular connections in the placenta. And again, I think of it as sort of a shunting process.

So we call the twin that's giving blood away or having a decreased blood supply the donor. And then the twin that's getting more blood supply is called the recipient. It tends to be volume overloaded. It's important that the fluid is discordant for both twins or abnormal for both twins. So you have to have polyhydramnios and oligohydramnios in order to call it twin to twin transfusion syndrome.

There's a staging system for twin to twin transfusion syndrome. It starts with stage one where you have the presence of polyhydramnios and oligohydramnios. Stage two is absence of the bladder in the donor. Here we're seeing a shift of the blood supply decreasing to the point where the kidneys are producing less and therefore less urine. The next is a critical doppler abnormality. So once you're to the point where you're suspecting twin to twin transfusion syndrome, you wanna be looking at doppler in these pregnancies. Stage four is the presence of hydrops, and stage five is the death or demise of one or both twins. Sometimes you can kind of skip a step, so you want to go with the finding that's the highest stage. So you may see critically abnormal doppler with a presence of a bladder still in the donor. And then you would still call that a stage three.

So this is another one of these pregnancies where you can see that there's a large field of view and without even showing you any measurements, you can tell that there's too much fluid. So you can't even tell which twin it belongs to, but the fetuses are not filling up the space of the amniotic fluid within the uterus. So if I see this kind of finding early in the pictures in the exam, I'm already thinking that I need to look carefully for the possibility of abnormal fluid.

So for stage one, we've got polyhydramnios of one twin and oligohydramnios of the other. So I'm showing you oligohydramnios of the donor twin on the left with a pocket of one centimeter. And on the right you can see that the bladder is still present within the donor twin.

Evaluating fluid can be a little bit tricky and is very important because as I've told you, it is the hallmark and the definition for whether this syndrome is present. So you can have something called a stuck twin when amniotic fluid is low or absent and then the amnion kind of shrinks down along this twin. And you can see that in this example where the donor twin is along the anterior surface of the uterus, it's sort of defying gravity and this makes it a little bit easier to see that this twin has no fluid and is stuck.

However, it doesn't have to occur in this position and it can be a little bit trickier when the fetus is stuck in a more dependent position. So this is an example of where you can get tripped up. If you see this membrane here that's sort of in between the abdomen and one of the extremities of this fetus, you may think that you can measure the DVP from that level to the posterior wall, the uterus, when in fact when you do some other scanning and you realize that the other twin has polyhydramnios, this is really the whole amnion wrapped and sort of collapse along itself. So there's really no fluid in this area and this fluid here belongs to the recipient twin. So you don't wanna make this mistake.

So with a little bit of careful scanning and looking for the other findings, you can see that in fact when you change the angle slightly, there are two pieces of the membrane here. There's this little triangle of fluid, and the rest of the fluid deep here actually belongs to the recipient twin. So this is a very small pocket, so this is in fact oligohydramnios. So you wanna look for these little corners, these little triangles of fluid when you're suspecting that there's potentially oligohydramnios. And another clue is there's gonna be very limited freedom of motion of this donor twin, whereas the recipient with polyhydramnios often is really moving around a lot within the uterine cavity.

So here we are back to this large field of view on the left. Again, this is a twin pregnancy where without even showing you the numbers, I'm gonna convince you that there's polyhydramnios of one of the twins. But when we measure the deepest pocket here, it's only 6.5. So we've said that the definition of polyhydramnios is eight centimeters. But I wanna caution you that when you're early in the second trimester or so, about 16 to 18 weeks, you can have twin to twin transfusion syndrome with a pocket that's less than eight centimeters. So you wanna think about this qualitatively, like I said, if you're looking on the left, you can see that there's too much fluid for one of these twins. So this number alone shouldn't talk you out of the diagnosis of twin to twin transfusion syndrome. The other twin in fact had low fluid in the absence of a bladder. So just be aware that in the early second trimester, this can occur.

The next important finding to make is absent bladder. So on the left we've got a twin with polyhydramnios, and then we have a stuck twin along the lateral wall of the uterus. And here we can see the umbilical arteries with color and there's no appreciable fluid in the region of the bladder. So these are tiny structures and it can be a little bit difficult, but if you're having to look very hard for a bladder, and if you've come back to this more than once during the exam to allow for the normal emptying of the bladder that can occur over time and you're not seeing a bladder, you wanna call this stage two twin to twin transfusion syndrome.

There is a tendency sometimes to under stage and call stage one 'cause you see a little area, you want it to be a bladder, but really if it's that difficult, you want to call it stage two.

Stage three is critically abnormal doppler. So you have to have some index of suspicion that this pregnancy is abnormal and then perform a doppler. And it's not just any abnormality, it's a few specific criteria. So the first is absent or reversed diastolic flow within the umbilical artery. The second is absent or reversed a wave in the ductus venosus. And the third is pulsatility within the umbilical vein.

So on the left we have a normal umbilical artery, so you can measure the systolic to diastolic ratio, but if you just look at the appearance of this waveform, we've got presence of diastolic flow in the forward direction. It's not even really necessary. This is what a normal waveform looks like. And you can contrast this to the image on the right where there's absence of diastolic flow seen at the end of diastole. And this would be an abnormal doppler and would be considered stage three.

The ductus venosus can also be a little bit difficult to image for a novice, but if you look here, you can see it in either the coronal plane as a little vessel connecting between the umbilical vein and the inferior vena cava. And then if you look in either plane, but this is shown in the transverse, turn your scale up and you're gonna see this aliasing little focus and this is what the ductus venosus looks like. So if you see that area, it's very classic in the right spot. And this is where you wanna put your cursor for analysis.

So this is a normal ductus venosus waveform. On the left you can see this is a very characteristic appearance of the waveform. Nothing else really looks exactly like this. And this area is called the A wave. So it's the deeper part of the or the indented part of the waveform. However, notice how it stays way above the baseline. So it's almost half the distance of the entire waveform. And this is normal. So this can be deepened, but if it's actually absent or reversed like we're seeing in this example, which is a inverted picture of the ductus venosus, this is an abnormal finding.

The umbilical vein is normally has a flat waveform. However, in the setting of stage three twin to twin transfusion syndrome, you can sometimes see this pulsatility. So here's an example of stage three twin to twin transfusion syndrome. We've got reverse arterial flow during diastole in the umbilical artery. And we've got a reverse a wave in the ductus venosus.

Stage four is hydrops. So you can see there's fluid in the abdominal cavity or ascites in this twin as well as anasarca edema. And stage five is demise. So here we can see that there's no color flow or beating heart in this twin. And although really functionally the vascular connections have now been changed or severed because of the lack of a beating heart in one twin, it's important to diagnose that this was attributable to twin to twin transfusion syndrome because of the potential sequela that can occur to the surviving twin.

So sort of putting this all together, this is a case that would be daunting, but with the tools and the things we've talked about, it's something that I'm confident you'd be able to recognize if in the rare instance you're faced with this. And I've seen this at least twice now in my time in doing this.

So here you can see that on the left we have two fetuses, A and B. And already I'm showing you that there is polyhydramnios of one of these. I'm not showing you the membrane, but it's measuring about nine centimeters. And here's the placental mass seen anteriorly. However, you may have already noticed there's actually a third fetus here in the lower corner of this uterus. There's a somewhat thicker membrane that's seen lying over top of it, relatively normal appearing fluid. So this fetus is filling up the space between the membrane and the uterine wall, and there's a second placental mass. So what we're looking at here is triplets. And so this is actually twin to twin transfusion syndrome between these two triplets. And then we have a third fetus over here that's essentially got its own placenta and its own membrane. So this is a dichorionic placenta one two and triamniotic three amnion pregnancy.

So briefly, treatment options for twin to twin transfusion syndrome often involves fetoscopic laser. So that's where you can actually go in and surgically sever these vascular connections between the twins. This is usually done for stages two to four, so that's why it's so important to correctly identify the stage and the outcomes can vary. But the quoted survival is about 80 to 85% of at least one twin. So this can be helpful or optimistic in the setting where they're both potentially at risk for demise.

Another treatment option is amniotic fluid reduction, but this is sort of a palliative approach and doesn't fix the underlying physiological abnormality. And in some selective cases, radiofrequency ablation of one twin may be considered.

It's important to remember that complications can occur. So I work at a tertiary care center where we perform these procedures. And sometimes the referrals sort of come with expectations of an intervention. It's really important to carefully weigh what the options are for each case and know that it is possible to make things better, but also to make things worse and that maternal or fetal complications can occur. So we really wanna make sure we do it when it's most appropriate.

So this is an example of a good outcome following a laser treatment. So here on the right hand side, right before the procedure, you can see there's no bladder in the donor. Here are the umbilical arteries. And immediately following the laser surgery, the bladder has returned in the donor. So we see that sometimes and it's really encouraging. Within about 24 hours here we see normalization of the fluid pockets within both twins.

On the contrary, you can get complications. So here is a very abnormal appearing brain sonogram. We've got hydrocephalus and hemorrhage within the ventricles. And MRI is quite complimentary, especially in more subtle cases, that we can use to look for changes in the brain secondary to ischemia. And here we can see the same hydrocephalus with T2 dark signal intensity from the intraventricular hemorrhage.

Another potential complication is ruptured membranes. So here we see the amnion is separated from the chorion. And another thing that can occur is you can inadvertently interrupt the septum or the membrane between the twins. This may then appear as normalization of the amniotic fluid, as potentially from the correction of the vascular abnormality, but it may be in fact from an inadvertent new communication. And this can be suspected when the fluid normalizes right away within the time of the surgery, as it would normally take more time if it was a result of changes in the vascularity.

Polyhydramnios Affecting the Recipient-Like Twin (PART)

So a similar syndrome that you may see is called PART or polyhydramnios affecting the recipient-like twin. And this differs from twin to twin as its name in that there is polyhydramnios but there is no oligohydramnios of the donor or the donor-like twin. And it's important to monitor these 'cause they still have a high progression to twin to twin transfusion syndrome. Expecting findings would be normal bladder for both twins, normal fluid, and you still can see weight discordance quite often. Again, there's a lot of overlap between these syndromes.

So here's a large field of view image of a twin pregnancy. I'm not showing you the numbers, but you can see that the twin on the left has way too much fluid. So this is polyhydramnios, but we see the amnion here and the twin on the right does not have oligohydramnios. So without measuring it, this is more than two centimeters. So the fluid is normal.

On the left, in the same case, you can see that for one twin the deepest pocket is 11, so that's polyhydramnios. And for the other it's six, which is normal. And there are normal bladders seen for both twins. Oftentimes there's an arterial to arterial anastomosis and it's worth looking for. This again, it gives you some potential prognostic information for the pregnancy, although it is not completely definitive.

Twin Reverse Arterial Perfusion Sequence (TRAP)

And last I wanna briefly talk about TRAP or twin reverse arterial perfusion sequence, also known as acardiac twin. So this is quite different than the other syndromes we've talked about, but again, has that uniqueness where it can only occur in the setting of a monochorionic pregnancy.

So here we have one twin, which is pretty grossly morphologically abnormal. So the main clue is there's no beating heart within this twin. You can get quite a lot of formation of some of the structures such as the abdomen and the extremities. But if you go through your checklist of normal anatomy that you're gonna look for as required by any second trimester ultrasound, this should become apparent that this is not a normal twin.

The hallmark of this is the reverse arterial perfusion. And I'll show you an example of that. And often they lack a cranium more than the lower part of the body, which is more formed. The other twin appears normal. The problem is that it's the normal twin that's perfusing this abnormal twin. So if you had a separate placenta for a fetus that didn't have a beating heart, it wouldn't be able to be sustained. However, this tissue in this abnormal twin is perfused and it can even grow with time. And that's because of the perfusion in these vascular connections in the monochorionic pregnancy. And this is stressful on the normal twin and can potentially cause high output cardiac failure and is why they need monitoring and potential treatment.

So this is a pretty characteristic appearance of a TRAP. I'm just showing it in one plane here, but we see soft tissue and there are some osseous structures seen centrally. Notice we're taking measurements and determining the volume can affect the treatment.

Here's the classic reverse arterial perfusion. So in a normal twin with a beating heart, you expect the arteries to be pumping the blood towards the placenta. And so we're gonna have a forward waveform. However, this fetus does not have a beating heart. The flow is coming from the other twin and moving in the opposite direction.

So in one study, determination for when you need to intervene on a TRAP was shown to be for one that has a volume of less than 50% of the normal twin conservative management can be taken. And then if the acardiac twin gets to be equal to or greater than 50% of the volume of the normal twin, treatment is usually preferred with radiofrequency ablation to prevent the potential cardiac failure for the pump or the normal twin.

This is just a schematic of what this looks like. So we're putting a radiofrequency ablation probe into the uterus and ablating the vasculature at the level of the umbilical cord. And again, remember that this is invasive. So that's why we wait until we think it's absolutely necessary to do this procedure. But these tend to have pretty good outcomes. The survival for the pump twin is equal to or greater than 85%.

Conclusion

So in conclusion, for monochorionic twin pregnancies, the most important thing you can do first is accurately determine the chorionicity. You wanna do this as early as possible, and I hope that you have some new tools to help you most accurately do this.

In order to look for these abnormal syndromes and monochorionic pregnancies, you wanna look for cord insertions, deepest vertical pocket presence of bladder and doppler abnormalities. And this will help you to potentially not miss these syndromes, including unequal sharing, twin to twin transfusion syndrome, PART and TRAP.

Thank you very much.