Describing a pregnancy using the word “twins” alone is not enough. The number of placentas that a twin pregnancy has is critical in determining the best way to safely care for the pregnancy. Close to 75% of all twins are fraternal, where each twin will always have its own placenta (dichorionic). The other 25% are identical twins, and among these, three out of four (75%) will share a single placenta (monochorionic). Perinatologist and twin expert, Dr. Larry Rand, discusses the importance of early diagnosis and frequent monitoring of monochorionic twin pregnancies in particular to ensure any potential complications are addressed.
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Hi, my name is Larry Rand. I'm a high risk obstetrician or perinatologist, and I specialize in complicated pregnancies, particularly complicated twin pregnancies. When you hear that you're having twins, the very first piece of information you wanna know. After congratulations, of course, is what the number of placentas is. Are there also two placentas, or is there just one placenta that both twins need to share? The word chorion in Latin means placenta. So when each twin has its own placenta, that's a dichorionic twin pregnancy. If both twins need to share a single placenta, that's a monochorionic twin pregnancy. This takes us back to the very beginning, because there are 2 ways that you can end up with twins. The first is that you actually ovulated 2 eggs, and each egg is fertilized by a sperm. That means you're going to end up experiencing basically 2 pregnancies in one uterus at the same time. We call this fraternal twins because they're really two siblings, 2 genetically different eggs at the same time. They can both be boys, they can both be girls, or they can be one of each. Importantly, because they start out as two entities, they each have their own amniotic sac, their own placenta, and basically are just experiencing crowding in the uterus at the same time. But they've each got their own set of resources from which their nutrition, oxygenation, and growth will come from. The second way that you can end up with twins is by ovulating just one egg. That egg is fertilized by a sperm, but then not long after being an embryo, it splits right down the middle from one into two. Splitting from 1 to 2 is quite an amazing thing and is how you end up with identical twins. What's really interesting is that if that splitting mechanism happens really quickly and efficiently, then they'll split all the way down the middle. Separate fetuses, separate amniotic sacs, and separate placentas. Much more commonly though, when you are splitting from one into two, it doesn't happen quite that efficiently or quickly, and the placenta is the last to split. And as a result, the fetuses will split, the amniotic sacs will split, but they'll end up sharing a single placenta. Unfortunately, sharing a single placenta can pose quite a number of challenges. Imagine a two family house, but each apartment doesn't have its own plumbing, and it's possible that 1 may take more hot water and not leave enough for the other. One set of shared plumbing can pose quite a number of challenges. The best time to determine monochorionicity or dichorionicity is in the first trimester. The earlier the better. If ultrasound is done at 14 weeks or less, it can tell you with nearly 100% certainty that the twins have their own placentas or are sharing a single placenta. After 14 weeks, it's still possible to tell if they're monocryonic. It's just a little bit less clear. Monochorionic twins can have a whole host of issues that come up in regards to how their circulations are connected within this one placenta and how they actually share the resources in this one placenta. That's why it's critical that they're followed very closely with ultrasound and in fact much more frequently than they would be if they were dichorionic twins again. It's critical that from the very beginning you and your doctor know whether your twins each have their own placenta or are sharing a single placenta. If you have a monochryonic twin pregnancy, you'll be able to set up surveillance and ultrasound and a management plan that's very different than a dichryonic twin pregnancy and will help make sure that you get early diagnosis and treatment of any complications that might arise.
