Mark P. Johnson, MD, MS: Sometimes families come to us with the diagnosis of TTTS, and it turns out to be something more like Selective Intrauterine Growth Restriction or sIUGR. TTTS and sIUGR are related because they involve the same common shared placenta between the two babies. What differentiates them is the vascular connections between the two babies.

Jack Rychik, MD: Often the differences between sIUGR and Twin-to-Twin Transfusion Syndrome can in fact be very subtle. Through the use of Doppler ultrasound, we can help tease out the facts that support one particular diagnosis versus another. And through Doppler echocardiography, we can identify the direction of blood flow the velocity of blood flow, and also determine patterns of blood flow.

Mark P. Johnson, MD, MS: One of the things that we look at, of course, is cardiac changes. In TTTS, there is a very characteristic progressive series of changes that happens in the larger twin, due to the extra volume that comes from the smaller twin to the larger twin. 

Jack Rychik, MD: Blood is exchanged from one twin, the donor, into the recipient, and then as a consequence, there's a huge cascade of hormonal changes that take place, that then brings about the cardiovascular manifestations in the recipient.

Mark P. Johnson, MD, MS: In selective IUGR, there isn't this transfer of volume. You know, there's not this net shift of volume from one twin to the other. It's much more balanced. But that balance, you know, can be a tenuous balance. 

Jack Rychik, MD: In the smaller twin, there is an abnormality of placental sharing. 

Mark P. Johnson, MD, MS: The smaller baby has a much smaller portion of the placenta.

Jack Rychik, MD: And the resistance in the umbilical artery of that particular twin, is much higher than normal, resulting in alterations in growth in the smaller twin. 

Mark P. Johnson, MD, MS: Selective intrauterine growth restriction has been recognized for a long time, but it's only been in the last decade that we've been able to see that there are different forms of it.

There's a milder form, which we call type 1. There's a more severe form that we call type 2, and then there's a new entity that's really only becoming understood, that's come out in the last few years. It's the type 3, selective IUGR. So if we were to look at a normal placenta, there would probably be a large number of connections between both fetuses.

There's kind of a sense of balance there, so that shifts in blood in one direction would be offset by shifts in blood in the other direction. In type 1 selective IUGR, what you notice is perhaps a 60/40 distribution of placental area, but you see a decrease in the number of connections. That means that there can't be as dynamic a shift in blood volume between the two twins.

When you go to the type 2 selective IUGR, you start to see a much smaller portion of placenta for the one that develops the intrauterine growth restriction. You also notice that the number of vessels, again, decreases even more. They tend to be balanced so that the number of artery to vein connections from the smaller to the bigger is still offset by artery to veins in the other direction, but the numbers are much, much fewer.

And so again, it's this idea of dynamic sharing that is more restrictive, and that forces the fetus with the smaller portion of placenta to really try to survive on what it's got as far as placental mass. And the less volume of the placenta it has, the more it struggles. In the type 3 selective IUGR, they have just small, small proportion of the placenta.

There tends to be a higher proportion of arteries connecting to veins, from the normal baby to the smaller baby, but it's a very small placental area, and that's led to the concept of rescue transfusion, the larger baby being able to send blood to the other side of the placenta. And so those connections are absolutely vital for the smaller baby's survival.

The other characteristic feature in type 3 selective IUGR is a very big artery to artery connection. There can be rapid shifts in blood in either direction. These high volume shifts of blood from one baby to the other, result in the blood pressure going up and going down and going up and going down, and that appears to result in injury.

So while this connection is vital to keeping the smaller baby alive, it can potentially result in a brain injury to the normal baby, and it can actually kill the smaller baby because of this just sudden rapid shift in blood pressure and volume. So in type 3 selective IUGR, it's a very, very dependent and very, very dynamic relationship between the two twins and the connections in the placenta.

Jack Rychik, MD: The science and the practice of managing twin complications such as twin to twin transfusion syndrome or sIUGR perhaps, in effect has only existed for about a decade. 

Mark P. Johnson, MD, MS: Understanding the TTTS story has been vital in our understanding of how to differentiate that problem from the selective IUGR problem.

Jack Rychik, MD: High volume exposure to these patients is critical in being able to learn what to expect. 

Mark P. Johnson, MD, MS: That high volume, that experience, and that collaboration is what makes us a really good place and allows us to really tailor management for each individual patient to optimize and try to achieve the best outcome possible.