Monochorionic twins are identical twins who share a single placenta between them. This occurs in nearly 75 percent of identical twin pregnancies. Two of the more significant complications that arise in these pregnancies, twin-to-twin transfusion syndrome and unequal placenta sharing, are discussed by perinatologist and twin expert, Dr. Larry Rand.
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Hi, my name is larry rant. I'm a high risk obstetrician or perry natale. A gist and I specialize in complicated pregnancies, particularly complicated twin pregnancies. Today we're going to talk about monaco chorionic twins, which are the types of twins that are most likely to have complications and issues that arise during the pregnancy. Let's start with some background information about mono cryonic twin pregnancies. So I'm going to start by drawing a uterus. So in this drawing your head is over to the left and the cervix is over to the right. And the cervix through which the baby would be born opens up into the vagina. And I'm going to add a placenta in the back. But remember the placenta can be anywhere in the uterus because the uterus is really just a hollow sphere and the placenta can implant anywhere inside that surface. So the placenta is kind of like a big sponge that sticks to the inside of the uterus and it acts as an interface between the babies and the mom. So mom sends her blood into the placenta and then the baby's access the blood and all of the oxygen and nutrients in it through their umbilical cords. The placenta is essential. It's the key to the baby's existence in the uterus really. It's the only way that they can get food and support. In fact, it's important to understand what the word monaco chorionic means, korean is just the latin word for placenta. So monaco chorionic is one placenta. So a mono chorionic. Twin pregnancy is a twin pregnancy. Two babies who need to share one placenta. Normally one placenta is really meant to support just one fetus. But if two babies need to share a single placenta, things can become unbalanced. I often joke that every placenta is like a snowflake. There's no blueprint, there's no map where the umbilical cord inserts into the placenta will be different every single pregnancy. Yeah. For the sake of simplicity I'll draw the umbilical cord right in the center of this placenta. For one baby and then right at the margin or the edge for the other baby. Think of the umbilical cord like a tree trunk that sort of sits in the soil and the roots are all of the blood vessels that come down from that umbilical cord. So each baby's cord inserts into the placenta and then spreads out its blood vessels like a network. Big problem is that there's just one placenta and there's no separation between The two Fetal circulations or the two sets of roots. And so not surprisingly some of the blood vessels will find each other and connect. So there are two key issues that are set up by sharing one placenta. The first is that because the blood vessels connect the babies can transfuse blood back and forth. And that's actually okay. They have the same blood type, their identical. It's okay as long as it's done equally and each baby sends a certain amount of blood and gets a certain amount of blood back in return, 90% of the time. Monochromatic twins do that without a problem. But 10% of the time. The way the blood vessels connect, don't allow the babies to transfuse the blood back and forth in an equal way. And that results in a twin to twin transfusion or T. T. T. S. Twin to twin transfusion syndrome. The second issue has to do with how to babies share this one placenta, how they share the resources and how that affects their size and their growth. So if you think about this one placenta how do we divide it up? So the key really is where the umbilical courts insert. If you look at this example we drew one umbilical cord in the center and one at the very margin. Now if you draw yourself an imaginary line between the two umbilical cords, you very quickly see that one fetus has two thirds of the placenta available to it, but the other fetus has just a third of the placenta available to it. So what follows is exactly what you might expect. The fetus that has the smaller amount of placenta available to it ends up being smaller while the other fetus who's got two thirds of the placenta grows perfectly normally. And the result is what we call unequal placental sharing. There are two very different issues resulting from this one shared placenta. One has to do with how the blood vessels connect and the blood going back and forth. The transfusion issue that's all occurring under the surface. The second issue is about how to babies share the resources of one placenta, and that has to do with where each umbilical cord inserts and the resulting share of the placenta that each baby has. This is why it's so important to have frequent ultrasounds in a mono chorionic twin pregnancy. It's the only way that we can keep tabs on how the babies are growing, whether there are any signs of transfusion and what to do next.
