Monoamniotic Twins

A twin pregnancy can mean twice the excitement and twice the fun for a mom-to-be. After all, what can be better than welcoming two bundles of joy? Unfortunately, twin pregnancies can also be quite challenging at times, causing twice the morning sickness and twice the fatigue. It can also be particularly stressful if your twins happen to be monoamniotic. Monoamniotic twins are quite rare, but require special care and monitoring. Here is some essential information if you are expecting monoamniotic twins.

What are Monoamniotic Twins?
Monoamniotic twins are identical twins that develop inside the same amniotic sac. Also known as MoMo twins (Monoamniotic-Monochorionic), monoamniotic twins are always identical. These share a placenta within their mother’s uterus, but have two separate umbilical cords for nourishment. Monoamniotic twins are rare, occurring in approximately 1 in 35,000 to 1 in 60,000 pregnancies. Monoamniotic triplets can also develop, but this is extremely rare.

Unfortunately, monoamniotic twins are at great risk for health complications due to the close proximity of the two umbilical cords in the amniotic sac. This makes it particularly easy for the twins to become entangled in each other’s cords, or to compress one another’s cords, endangering their oxygen and food supply. The survival rate for monoamniotic twins is approximately 50%.

Why Do Monoamniotic Twins Form?
No one is really sure why monoamniotic twins develop. Identical twins (also known as monozygotic twins) develop from a single fertilized egg that splits at some point after fertilization. Typically, this split occurs a few days after fertilization, just after the placenta forms but before the development of the amniotic sacs. These twins share a placenta but have individual amniotic sacs.

However, sometimes the fertilized embryo does not split until after the formation of the amniotic sac. As a result, these identical twins share both a placenta and an amniotic sac.

Recent research has shown a correlation between the number of yolk sacs present in pregnancy and the number of amniotic sacs twins will have. The yolk sac forms just after conception occurs and is thought to play a role in nourishing the developing embryo. With twins, there can be one yolk sac or two yolk sacs.

It has been found that twins with two yolk sacs are much more likely to have two separate amniotic sacs. On the other hand, twins with just one yolk sac seem to be more likely to have only one amniotic sac. Unfortunately, it is very difficult to detect the number of yolk sacs present during pregnancy, because the yolk sac disappears in the initial weeks of the first trimester.

Complications Associated with Monoamniotic Twins
Unfortunately, monoamniotic twins face a lot of potential health hazards throughout pregnancy. Many of these can be life threatening, so it is important to make sure that you receive specialized care if you are experiencing this type of twin pregnancy. Possible complications include:

  • Cord Entanglement: Cord entanglement is one of the main complications associated with monoamniotic twins. Because the twins have no amniotic membrane separating them, their umbilical cords can easily become entangled. This can hinder fetal movement and development. Additionally, if the twins become tied together in the uterus, one twin may become stuck in the birth canal during labor and delivery, which can be life threatening. Cord entanglement happens to some degree in almost all monoamniotic pregnancies.
  • Cord Compression: Cord compression is another life threatening condition common in monoamniotic twins. As the twins move around in the amniotic sac, it is possible that one will compress the other’s umbilical cord. This can prevent vital nutrients and blood from traveling to the other baby, resulting in fetal death.
  • Twin-to-Twin Transfusion Syndrome (TTTS): Twin-to-twin transfusion syndrome can occur in monoamniotic twins. This syndrome happens when one twin receives the majority of the nourishment in the womb, causing the other twin to become undernourished and sickly. Typically, TTTS is diagnosed by examining fluid levels in the two amniotic sacs. But because monoamniotic twins only have one amniotic sac, diagnosis of TTTS is much more difficult. Diagnosis rests on comparing the physical development of both of the twins.
  • Preterm Birth: All monoamniotic twins are born premature. This is because full-term pregnancy is deemed unsafe by most medical professionals. The risk for cord entanglement and compression simply becomes too great after 34 weeks. For this reason, all monoamniotic twins are delivered by cesarean section at 34 weeks. However, many monoamniotic twins experience life-threatening complications as early as 26 weeks, and need to be delivered immediately. Preterm delivery is often associated with a number of life-threatening conditions.

Treatment for Monoamniotic Twins
Unfortunately, there is no treatment that can reverse this pregnancy condition. There are also few medical treatments available to improve the situation. An experimental drug, Sulindac, has been used to in some monoamniotic twins. This drug lowers the amount of fluid in the amniotic sac thereby reducing the amount of fetal movement. This is thought to lower the chances of cord entanglement or compression. However, this drug has not been studied in a large number of pregnancies and its potential side effects are unknown.

The best treatment for monoamniotic twins is to have regular and aggressive fetal monitoring. You should be sure to see a perinatologist (also called a maternal-fetal medicine specialist) regularly throughout your pregnancy. You should have twice-weekly monitoring of fetal heart rate and movement, particularly after the 26th week. After the 28th week, many women enter the hospital, so that they can be constantly monitored. Aggressive monitoring can help to lower the risk of fetal death considerably.

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