Multiple Gestations: The Terrible Twos - HD

Introduction and Disclosure

My only disclosure, I'm a medical editor for the Amherst content at Elsevier, and I have the lofty goal here.

What I really want everybody to totally understand by the end of this is how to determine chorionicity and amnionicity and what that means for pregnancy outcomes.

And to start understand chorionicity.

And amnionicity really gets to the heart of what's going on in multiple gestations.

Importance of Chorionicity in Twins

Starting with twins, what really matters.

Now, only 1% of pregnancies are twins, yet 10% of perinatal morbidity and mortality are related to multiple gestations.

And what does really matter?

Obviously, anomalies growth is really important in that chorionicity is extremely important.

And if you have a normal scan live, two live embryos at six weeks, if you're monochorionic, you only have about a 40% chance of making it.

And by 12 weeks, if you're monochorionic, you still only have about a 75% chance, roughly of making it.

So it really does matter.

Embryology of Fetal Membranes

The first part of this, I really wanna review the membranes and where they come from and what they look like.

As you recall, fertilization occurs in the tube.

We start dividing.

We form this blastocyst, and then this blastocyst buries itself within the endometrium.

And when it does that is what we call the intradecidual sac sign.

Now, we already heard that some of these signs that we learned are not always specific, but I think it really does help to use them when we talk about these milestones.

So first of all, the word decidua is endometrium.

Decidua belongs to the mom.

And so here we see the intradecidual sac sign, and then that goes on to the double decidual sac sign.

And we can see the basalis here.

The capsularis goes over the top, the parietalis on the outside, and this hypoechoic layer is actually the endometrial cavity.

What remains of it?

Quickly, this wall will coapt that wall, and you'll never see this again.

And this is a transient finding.

So that's the decidua.

The decidua is the endometrium.

But let's get to the meat of the matter.

And that is what's going on on the inside of this.

And we have the chorionic sac, that's our first fetal membrane, the chorion.

So the chorionic sac and the gestational sac are the same thing.

If you say, I see a gestational sac, you're seeing the chorion sac, you're seeing this nice thick, juicy membrane out there.

And you can see it on this specimen here.

That's the chorion.

That's the chorion.

It's big and thick and juicy.

And that's gonna be a recurrent theme as we go through this.

Now, this chorion is really very smart.

This is, it's looking for food.

That's its job.

And so where is this gonna grow?

Well, it's gonna grow towards the base here, towards the myometrium.

This is where the blood is coming from, and it's gonna retract over the top.

And we're left with the smooth chorion or the chorionic membrane.

And then we have this frond-like, or villous chorion at the bottom, which we can see here in this embryologic specimen.

All right, so now what's inside the chorion?

We have the amnion, that's our outer layer.

It belongs to the embryo.

And the first thing we're gonna see is the yolk sac.

We love the yolk sac, don't we?

It's so nice and round.

It's easy to see.

It tells us a hundred percent we're dealing with an intrauterine pregnancy.

The yolk sac is our friend, but let's not think that that's the thing that develops first.

There is absolutely an embryo in there.

It's just below our ability to resolve.

And on this photo micrograph, there's the yolk sac.

The embryo grows on top of the yolk sac, and it has an itty bitty little thin amnion around it.

Way too small for us to see.

And then all this thick frond-like juicy chorion out here.

So we move forward a little bit, and we have what Ann had mentioned the diamond ring sign.

We start to see a little bump on this yolk sac.

And that's the embryo.

And there's absolutely a little amnion around that thing.

We just can't see it at this point, but it's gonna grow.

And we're gonna have our amnion right there.

So small little amniotic cavity, large chorionic cavity at this time.

As we get further out in the first trimester, once again, big thick chorionic cavity, thin little amniotic cavity, that's your amnion right there.

It gets closer and closer to the chorion, but they don't fuse until the early second trimester.

But after that point, we don't see a separate chorionic cavity anymore.

We just call it the amniotic cavity.

All right, so to just encapsulate this, we have the three layers of decidua that come from the mom.

That is the endometrium, the parietalis, the capsularis, and most importantly, the basalis.

The membranes around the fetus are the thin little amnion that starts really close to that little embryo and grows and grows and grows through the first trimester.

And then we have our big thick juicy chorion, the smooth chorion or the chorionic membrane.

And then the villous chorion down here.

And the villous chorion with the decidua basalis of the fetus, with the decidua basalis of the mom makes the placenta.

All right, so the chorion is responsible for the placenta.

Let's keep this in mind and let's move on to multiples.

Types of Multiple Pregnancies

Now, multiples, we can have two types of multiples.

Everybody knows this.

The dizygotic or fraternal twins and the monozygotic or the identical twins dizygotic are actually a lot more common.

And these are two separate fertilization events.

And this will increase depending on age parity, family history, and some ethnic diversity.

Monozygotic though are fairly constant.

It's about one in 250 pregnancies and seems to be independent of those other things.

House Analogy for Chorionicity and Amnionicity

So I like to think of this as the uterus is the house.

All right?

And in this house, what do you want?

You want the most square footage you can get a nice swimming pool and a gourmet kitchen.

All right, so what's the kitchen in this thing?

The kitchen is the placenta.

Where did the placenta come from?

It's the chorion.

The chorion makes the placenta.

So if you have to share your house, okay, you really want it to be a duplex, right?

You want to be dichorionic.

You want two separate placentas.

The amnion in this analogy is the bedroom.

And so you want this nice thick membrane separating you and tastes.

This guy plays his music really loud at night.

You have this nice thick membrane.

Now, if you have to share a chorion, your monochorionic, you are sharing your food supply, you're sharing your kitchen.

Now, I know people have had roommates in your life.

You bought the pizza, the roommate came down and ate it all right?

That is twin-twin transfusion where one of the embryos, one of the fetuses, is really getting too much of the nutrition and one's starving over there.

But at least you're in your own bedroom in this situation.

And if you're monoamniotic, you're sharing the bedroom as well as the kitchen.

Put two little kids in the same bedroom within five minutes, they're gonna be in some sort of tussle in there.

And that tussle on a fetus is essentially getting their cords wrapped around each other.

And this is what we really worry about in a monoamniotic pregnancy, are these cord knots that you can get and they can cause issues.

So if you are dizygotic, by definition, you're going to be dichorionic, diamniotic.

And early on, this is so easy to tell and it's really incumbent upon the first person that sees this to make the diagnosis.

So clearly two separate chorionic sacs.

Look at that really nice thick chorion on each side, separating them.

You can see each chorion nice and thick, and we get into the second trimester, and sometimes we can see a separating piece of placenta going up between here, which we call the twin peak sign.

If we see it, it's great, but if you don't see it, you'd be stuck here.

So here we have, I mean, easy peasy, big thick membrane dichorionic.

This would be hard to tell.

All right?

And this is the same case.

If you are monozygotic, it really depends when you split apart.

If you, if we use our construction analogy again, if under the, if you notify the contractor before day three, you can have two separate kitchens and two separate bathrooms.

Excuse me.

Well, it is kind of a bathroom as well as a bedroom, I guess.

All right?

That's a combination.

But day four, sorry, the kitchen's been built.

You are monochorionic.

All right?

You can still have two bedrooms until you get to after about eight days, kitchen's built, bedrooms built, you are monochorionic monoamniotic.

And about after 14 days, you are conjoined.

Diagnosing Monochorionic Diamniotic Twins

So what are we gonna look for in a monochorionic diamniotic?

Well, remember where the embryo grows.

So early on we see we, there are two yolk sacs.

We know the embryo grows on the yolk sac and there's a little tiny embryo amnion there whether we can resolve it or not.

Now, Dr. Pretorius always likes to showcase where you get, you can be fooled here, but I think it is a very good general marker.

But obviously you're gonna follow up when you follow up one big thick chorion out there.

Two little tiny amnion going around each embryo.

Two yolk sacs, which I think are as a nice marker for this.

Monochorionic Monoamniotic Twins

Here we have a single amnion, two embryos inside a single yolk sac.

This is monochorionic monoamniotic.

People look at those and go, oh, look how cute.

They're so snuggling.

Well, there's a real problem.

And that is, look where their cords are.

Their cords are exactly side by side here.

Now I've seen them, like in this case where they actually, it almost looked like there was just a single cord because they were so entwined with each other.

But actually in some ways this is not so bad 'cause it gives it some stability.

Actually.

It's the knots that we worry about.

There was an article though that said maybe we worry about these too much.

They looked at 114 pairs of these twins and looked at the survival approaching 90%.

And they were saying that cord entanglement but really should be viewed more as a minor complication that really prematurity and other things are more important.

Now having said that, I certainly have seen cases where, boy, that looks scary.

And indeed it was 'cause there was a cord accident in utero.

And you can see the tangle of cords here.

There was an in utero demise.

You can see the thrombosed cord coming along here.

And the issue is, we shall see is if one demises, that really is a very big concern for the one that remains living.

And we'll look at that in more detail.

Higher-Order Multiples

The same principles could apply to the higher order gestation as far as naming.

So here we have big, thick separating membranes.

Each one has their own chorion their own placenta.

So this is trichorionic triamniotic.

Here we have a dichorionic triplet, okay, big thick membrane here with this guy.

Two yolk sacs, each of which will have an embryo and an amnion.

So this is diamniotic.

So in total, this is dichorionic triamniotic.

And this one, it is dichorionic diamniotic 'cause there's no membrane here.

Now that is correct, but I find it personally confusing.

My brain gets a bit of a cramp.

Now what does that really mean when I describe these, I'll say there's a triplet pregnancy with a singleton and diamniotic pair, a triplet pregnancy with a singleton and a monoamniotic pair.

And then I think people get that better.

Here we have a very bad situation, which is a monochorionic thin membranes in between triamniotic.

And we all know what this one is, right?

No question about it.

That's a bad day.

An ultrasound, wait till they come back for their anatomy scan.

All right, what trouble?

So we, we've actually spent a fair time talking about how we make the diagnosis to look at the chorion.

The amnion, what does that mean?

But here's what they can get into trouble if you're sharing your food supply, if you are monochorionic, we're gonna go over the three most common things, starting with twin-twin transfusion.

Complications in Monochorionic Pregnancies

Twin-Twin Transfusion Syndrome (TTTS)

The issue with twin-twin transfusion is you have an artery to vein anastomosis.

And what happens is this guy here where he should be sending half his blood supply, one of his arteries out into the placenta, it's going over here.

So he is not getting all of this blood oxygenated and get his nutrients back to him.

He is all anemic.

What happens when you have anemia?

Well, you're gonna decrease perfusion to your kidneys.

You're gonna decrease urine output.

You're not gonna see a bladder and you're gonna develop oligohydramnios.

This guy, on the other hand, is getting way too much.

What's he gonna do to get rid of all this extra fluid?

He's gonna diurese as much as he possibly can and he's gonna have polyhydramnios fluid is key to twin-twin transfusion.

These are the oligo poly twins.

You really need to have oligo and polyhydramnios to make this diagnosis.

And here we have a classic pair.

This is the donor.

He is very scrawny here.

He looks very emaciated, where the recipient looks fluid overloaded.

And you know, this extra food and fluid that he's getting, it's deoxygenated, it's depleted of nutrients.

So it really is doing nothing but hurting him.

This occurs, twin-twin transfusion occurs because there's this artery to vein anastomosis.

But it's important to realize that in almost any monochorionic pregnancy, there are indeed anastomoses and they're not all created equal.

Commonly you'll have an artery to artery like you see here.

They tend to be very superficial.

They're felt to be protective because they're bidirectional and they can balance out blood distribution.

And so these, we don't worry about.

What we are concerned about are these artery to vein anastomoses.

They usually dive deep into the placenta.

And these are unidirectional.

These are unidirectional.

So what happens, as I said, oligo and poly are the keynote things to see.

You can see the membrane start to drape around the donor here.

Often the fluid will look a bit more echogenic because it's concentrated.

It's kind of like this kid in his science experiment and sucking all the air out.

It's a similar thing that the membrane drapes around him.

So often you'll see this fetus in a odd fixed position and often you won't even see the membrane.

And these can be misdiagnosed as monoamniotic, but if it was monoamniotic, they should be floating and moving around.

But if you see 'em stuck in odd positions, that's your clue what's going on.

The staging, there's several staging systems.

The most one, a common one that we use is the Quintero, where we're looking at the bladder and the doppler.

So we start off with oligo poly.

You can see the membrane draped around here.

We look for the bladder.

If the bladder is absent, this is now a stage two.

By the time the doppler goes bad, you can see no diastolic flow here.

That is now a stage three.

And here we have reverse flow.

Reverse flow is often an ominous finding in these fetuses.

And then finally in stage four, you develop hydrops.

Now clearly this fetus is hydropic.

I wanna point out the donor twin.

Here again, you don't see a membrane because it is totally stuck.

It is shrink wrapped against the wall.

And by the way, notice you don't see a stomach, not 'cause there's esophageal atresia because this, there's nothing to swallow, there's no fluid in there to swallow.

And then in stage five, there's demise of one or both other systems incorporate other findings.

Most importantly cardiac findings.

And it's one of the most important things in the recipient twin, when they're getting so fluid overloaded, they develop a cardiomyopathy, they get tricuspid regurgitation.

And that is a one of the worst signs that you can see in the recipient twin.

So twin-twin transfusion up to 20% of monochorionic pregnancies, high mortality rate untreated.

Usually it's the donor who dies first, but if one dies, it is a bad portent for the living twin.

And so these are one of the things that we want to try to intervene on.

You could consider early delivery depending on what gestational age they've tried serial amnio reductions at septostomy.

But really the best treatment is a coagulation where they go in with a laser, you can see the ultrasound guidance and they essentially create a equator here.

And they've now are starting to do some more sub selective things.

But you're really trying to make this into a dichorionic pregnancy by getting rid of those anastomoses.

Discordant Growth in Twins

One thing that I wanna point I wanna get across here is discordant twins are not the same as twin-twin transfusion.

They're often discordant.

But you can have discordant twins and we'll get 'em sent all the time.

Look for twin-twin transfusion.

Remember, the fluid is the key to twin-twin transfusion.

You have to have oligo and poly.

By definition, plain old discordant have none of these unbalanced vascular anastomoses.

Our definition is greater than 20% difference in fetal weight or 20 millimeters AC you can also look at a ratio.

But in a monochorionic pregnancy, we should always consider this abnormal.

Okay?

If it's dichorionic, not so much.

We give them more leeway 'cause they're two separate fertilization events.

They're genetically different.

Where these are really should have similar growth potentials.

So here we have a growth curve and it's of these guys right here.

And the little chubster right here did just fine, but this guy, the troublemaker, and you can tell just by looking at him, he's gonna be a troublemaker.

His growth is dropping off and he is at risk for fetal demise at this point.

And so this is one, they wanna get steroids on board and get this out because now you're really treating the pregnancy because if this guy dies, it is really bad on the living twin.

So these guys continue to do well.

You can see at three they still have quite a growth discrepancy.

But the mom says he's still the troublemaker in the group.

So why, what could be causing this growth discrepancy?

And I really wanna encourage everybody.

Part of our guidelines now mandate that we look at the placental cord insertion.

And if you look at the placental cord insertion, often that's your answer in these cases because here it's inserting nicely in the middle of this nice placenta, this guy is a barely a marginal insertion hanging on at the end.

This one's getting about two thirds of the placenta.

This one's getting about one third of the placenta.

And so marginal and velamentous cord insertions may be responsible with unequal placental sharing for these growth discordant growth trapped twins.

Twin Reversed Arterial Perfusion (TRAP) Sequence

Twin reversed arterial perfusion.

Now what this is, is an artery to artery anastomosis and the artery will now have reversed flow in it of deoxygenated blood when it hits the trapped twin, the acardiac twin, where's arterial blood going?

It's going down to the lower extremities.

Nothing's going north here.

And so you get very poor development of the torso, no development of the heart.

So it's reversed flow in the umbilical artery.

So that is the absolute most definitive thing that we're gonna see.

Other things is this is often an amorphous globosus looking, kind of a blob.

Usually no head you can see no head on this one becomes very globosus in appearance when you look on the pathology specimen.

Here's the artery to artery.

And often they only have a single artery.

You can see no head development.

There was an omphalocele too.

But the absolute diagnostic feature is if you see reverse flow, arterial flow should never be towards the fetus.

The pump twin.

They can get into trouble and get into trouble early.

We look at ductus here and you can see we already have reversal of the A wave indicating strain on cardiac output.

Twin B, here we go.

And by 17 weeks, enormous by 25 weeks, there's the normal one.

Look how big this thing is.

And again, they all have this kind of blob look to them.

We really care about the pump twin, right?

We don't care about the acardiac twin and obviously they can have big issues.

Demise is very high.

As far as treatment, we're gonna try to preserve the normal twin interruption of blood flow is the best thing to do.

They've tried surgical ligation, they've tried laser.

Probably the best thing though is radio frequency ablation where you put a needle into the torso here and you pulse until their cessation of blood flow.

If you haven't, by the way, notice that this was a diamniotic separating.

So twin A actually did okay, but if you're monoamniotic, now you have an added problem because we saw cessation of blood flow in this one at 16 weeks.

We were watching this one.

But a sudden demise at 24 weeks.

And you can imagine what happened because this is like a tether ball on a string.

And that string got wrapped around the cord and thrombosed the cord of the living one.

This obviously is an uncommon situation, but if this happens again, we've decided we're gonna go in and ligate this one and set this one free.

Consequences of Fetal Demise in Monochorionic Twins

What happens with one demise?

We used to call this twin embolization syndrome.

That's not true at all.

I need to remind you of these anastomoses that are pretty ubiquitous in these monochorionic pregnancies.

So what happens is, if this twin dies, all of a sudden the pressure head in this artery drops and all of a sudden this guy who was just perfusing and they were nice and equal here, all of a sudden that blood is now going to the rest of the placenta and it's probably a bad hypotensive event and it causes hypoxic injury.

And the brain, heart and kidneys are very vulnerable.

And look at this case.

We know that two days before this they were alive and two days later, twin B already has edema.

By a week we have ventriculomegaly.

Look at the heart.

Okay, thick echogenic, there's an effusion and look at the size of that heart one week later.

So a big hypoxic event.

Conclusion

So first person to scan must determine chorionicity and amnionicity and be aware that we need to follow these monochorionic pregnancies.

Thank you very much.