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. As a result there is a need for fetal monitoring during the pregnancy. The survival rate for monoamniotic twins is now over 80%.

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.