Monochorionic twins have to share a single placenta but don’t always share it equally. Depending on where each fetus’ umbilical cord inserts into their shared placenta, one twin may get a smaller share of the nutrients and oxygen that are critical for normal growth and development. Perinatologist and twin expert, Dr. Larry Rand, describes the clinical markers of this condition and the importance of frequent ultrasound surveillance when monochorionic twins are found to be more than 20% different in size.
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Hi, my name is larry rand. I'm a high risk obstetrician or perry natale a gist and I specialize in complicated pregnancies, particularly complicated twin pregnancies. Let's start with the basics. This is a drawing of your uterus and in this drawing your head is to the left and the services over to the right. Now let's add a placenta. The placenta is essential to the baby's existence in the uterus and is how the baby will grow and get food mom sends her blood to the placenta filled with nutrients and oxygen that the babies need to grow and the baby's access those nutrients and oxygen through their umbilical cords. Each baby's umbilical cord inserts into the placenta. Think about the word mono chorionic korean is latin for placenta. Mono Chorionic refers to the fact that there's only one placenta for two babies. Now. Normally the way that a pregnancy functions is that each baby gets its own placenta. One baby, one placenta for normal growth. But in a mono chorionic twin pregnancy, two babies are expected to share a single placenta. Let's talk about something called unequal placental sharing. So in all twin pregnancies it's important to watch how the babies are growing. You'll hear a lot about each baby's size at various ultrasounds, particularly in a mono chorionic twin pregnancy. Both babies need to draw their nutrients and oxygenation from that one placenta. How the babies grow over time as an important marker. Now it's also normal for twins to vary in size and in fact we allow up to a 20% difference and consider that normal, but once the difference between the twins is more than 20%, we call that discordant for growth or size. And it becomes more important to watch very closely. In particular when we're watching a twin pair that's discordant in size, The situation is usually that one twin is appropriately grown, but the other twin is too small or lagging behind significantly. In fact, the difference in size between a pair of twins can be pretty accurately predicted by where the umbilical cords insert into that placenta. When you have to centrally inserted umbilical cords, we'd expect the growth to be pretty similar. Each twin has similar access to the placenta and it's a pretty well shared placenta. But when the umbilical cord one is in the central portion of the placenta, but the other is all the way at the edge or marginal or even more significant. Doesn't even insert into the placenta, but inserts into the amniotic sac or the membrane. Something called a vela mentis cord insertion, we'd be much more concerned about fetal growth and follow those twins more carefully. If a smaller twin is not quite getting enough from the placenta and is struggling. One of the signs that will notice on ultrasound is that the amniotic fluid volume can drop and become low when a fetus is not quite getting what it needs, its body tries to hold on to every bit of precious nutrition that it does have and preferentially sends blood to the brain and the heart and sort of puts the kidneys last. And so the result is you slow down how much urine you're making and amniotic fluid volume is really just baby urine. So when we are following a fetus that's smaller, seeing a low amniotic fluid volume is an important sign on ultrasound. A common misconception actually is when you see on an ultrasound that one twin has normal fluid and the other twin has low fluid might make you think that you're looking at twin to twin transfusion syndrome. But you're not treating this as twin to twin transfusion and lays during the connections between such twins is not the answer and it's not the right treatment. In fact, it could be quite harmful to the smaller twin because it disconnects the sometimes helpful connections, twins with unequal placental sharing often have a protective blood vessel connection between them two arteries that joined together or an arterial arterial anastomosis, which we abbreviate as an a connection. And its protective and actually helps prevent developing twin to twin transfusion syndrome. So again, really this is just an issue of growth and how the placenta is shared. The key in unequal sharing is actually to avoid adding additional risk by having an operation or surgery. It's critical that twins with unequal sharing are followed very closely with ultrasound, Looking at their growth every 14 days, looking at the amount of amniotic fluid in each sack and looking at the blood flow through each umbilical cord to the placenta. And if we do that, the average age of delivery is actually between 33 and 34 weeks, which is quite a good outcome. Mhm.
