Unequal Placental Sharing
Twin pregnancies come with certain risks and while chances are your pregnancy and delivery will be just fine, it pays to educate yourself about possible complications so you can spot symptoms and get yourself treated. Some twin pregnancies are at particular risk because the twins share a placenta. This is called monochorionic (MC) twin pregnancy.
By definition, MC twins are identical. Because they share a placenta, they also share vascular connections. In some 15% of all MC twin pregnancies, the circulation of the two fetuses becomes imbalanced. Sometimes this leads to a situation in which one twin, called a "donor" twin, is transferring large quantities of blood to the other twin. This is called twin to twin transfusion syndrome (TTTS).
While neither twin is malformed, one twin transfuses the other by means of a specific blood vessel that connects the two of them. The donor twin's artery dips into the placenta to take air and nutrients and then returns to the wrong twin. There is often a significant difference in the weights of the two babies and in the volumes of their respective amniotic fluids. TTTS is considered the most common complication for a MC twin pregnancy, but there are plenty of other problems which can occur.
Another MC twin pregnancy complication causing different-sized twins and differing levels of amniotic fluid is unequal placental sharing. In this condition, the twins have unequal access to the placenta. Because access is limited for one twin, his growth slows and he has less amniotic fluid. This situation is progressive and can lead to the smaller twin becoming stillborn. Because there are common blood vessel connections between the two fetuses, if one twin dies, the other is endangered. In unequal placental sharing, the bigger twin does not have an increased amount of amniotic fluid; however, unequal sharing and TTTS can coexist.
With the help of ultrasound examinations, your doctor should be able to gather the information needed to determine the seriousness of the situation. The doctor bases his evaluation of the problem according to how large a gap exists between the babies' weights and sizes. Your doctor will want to monitor the amount of amniotic fluid in the sac of the smaller twin. As the fluid level dwindles, the situation becomes more worrisome. A Doppler ultrasound can tell your physician a great deal about the blood flow in the umbilical cords of the two babies. In general, a high resistance pattern will be found in the smaller, weaker twin, while the larger twin's blood flow will be normal.
There is no treatment for unequal placental sharing. The important thing is careful monitoring of the situation and of the smaller twin. As the smaller twin becomes sicker, your doctor may want to hospitalize you to keep a closer eye on things. Your doctor may want to deliver you early. The scenario to avoid is that the smaller twin dies in utero, because this will lead to serious health issues in the surviving twin.
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