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In most cases, the biggest distinction people make between twins is whether they’re identical or fraternal (and even that separation can be confusing for some).

If up until this point twins have been a genetic mystery for you, please allow us to introduce you to some of the various types of twins. One of the most rare and complicated types are sometimes called mo/mo twins. Confused? Intrigued? Keep reading to find out more!

Mo/mo is a shortened form of monochorionic monoamniotic. Mo/mo twins are those who share both the chorionic and amniotic sacs.

Or to put it another way, since mono means “one,” a mo/mo pregnancy is one where there is one placenta and one amniotic sac for both babies.

Confused about how this works? Allow us to back up a bit…

Most people have heard mention of identical and fraternal twins before, right?

  • Identical twins (also called monozygotic twins) come from the same fertilized egg. They are created when one egg and one sperm meet as expected. Shortly after fertilization, the single egg splits in two. These twins are referred to as identical, because they share all the same chromosomes.
  • Fraternal twins (also called dizygotic twins) on the other hand come from two different eggs that are released close together and fertilized by two different sperm. They share only 50 percent of their chromosomes as other siblings do, and characteristics like hair and eye color or gender can be the same or different.

Got all that and wondering what that has to do with mo/mo twins? Well, believe it or not, twins can share a few other things besides chromosomes!

When it comes to identifying types of twins, classification depends on whether the fetuses shared or had separate chorionic and amniotic sacs.

Never heard of a chorion or amniotic sac? Don’t worry, you’re not alone, so here’s a quick recap and a few notes on why they are important.

While developing in the womb, the chorion is the outermost fetal membrane, the outer layer of what is known as the amniotic sac. The chorion works to connect the amnion, amniotic fluid, and the fetus to the placenta.

The amnion is the innermost fetal membrane. It helps protect the fetus and includes amniotic fluid. Similarly to the chorion, twins can either share an amnion or have one all of their own.

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Illustration by Alyssa Kiefer

One fun fact to keep in mind is that even though two babies may share the same placenta (i.e. one chorion for the pair), they can each still be in their own amniotic fluid if they have their own amniotic sac (i.e. two separate amnion).

This means that there are actually 3 different combination possibilities: mo/mo, mo/di, di/di:

  • Di/di twins (dichorionic diamniotic) each have their own individual chorion and individual amnion.
  • Mo/di twins (monochorionic diamniotic) share a chorion, but have separate amnion.
  • Mo/mo twins are those with one amnion and one chorion. Everything is shared!

If the twins are fraternal, they’re di/di and each is in their own little bubble. Di/di twins also account for about 30 percent of identical twins. Depending on the timing of the egg split, identical twins can be di/di, mo/di, or mo/mo. Mo/mo is rarest, accounting for only about 1 to 5 percent of all identical twins.

While all of this may seem like a lot to understand, the issue is that the more that the twins share while in utero the higher the risk to the pregnancy.

Knowing which type of twins a pregnant person is carrying can help providers to identify and monitor for any possible issues.

Mo/mo twins are typically identified by ultrasound fairly early in the pregnancy. The best ultrasound images for this identification happen within the first 14 weeks of the pregnancy when the placenta and amniotic sac are more visible.

For a mo/mo pregnancy, the ultrasound will show one placenta supplying blood to two fetuses. Both will also appear within the same amniotic sac, with no dividing line between them.

Diagnosis is also possible if there is evidence of umbilical cord entanglement. Early diagnosis is important so that the pregnancy may be closely monitored.

Any time someone is pregnant with twins there are additional pregnancy risks. Many revolve around intrauterine growth restriction, which is a fancy way of saying that there’s only so much room for two babies to grow!

Some complications that are more likely with a twin birth include:

Because the two babies are sharing so much in a mo/mo pregnancy, it can be extra risky. Some additional possible risks of a mo/mo pregnancy include:

  • Premature delivery: In a mo/mo pregnancy, cesarean delivery is typically recommended by 32 weeks to reduce risk.
  • Birth weight differences: If twins are growing at unequal rates, there can be issues of one twin restricting the other’s growth inside the placenta.
  • Amniotic fluid problems: Too little amniotic fluid can limit bladder size and movement. Too much amniotic fluid can lead to an enlarged bladder and increase the chance of heart failure.
  • Cord entanglement/compression: There is nothing to separate mo/mo twins, which means their umbilical cords can become tangled over time. This can lead to very early twin deliveries if there are any concerns, since an entangled umbilical cord can result in sudden death.
  • Twin to twin transfusion syndrome (TTTS): When the placenta is shared, as it is in a mo/mo pregnancy, there is a risk that one twin’s share of the blood volume and nutrients will be more than the other’s. This can cause issues for both the babies and may require monitoring, premature delivery, or surgical repair in utero.
  • Twin reversed arterial perfusion sequence (TRAP sequence): Occurring in only about 1 percent of mo/mo pregnancies, one twin is developmentally normal (the pump twin). The other twin forms without a functioning heart and many other body structures. The two are joined by a large blood vessel, and the twin without a heart receives all its blood from the pump twin. This causes the pump twin’s heart to work much harder than it normally would. The larger the twin without a heart becomes the higher the likelihood that the pump twin will experience heart failure or death.

There is no way to prevent mo/mo twins, so doctors focus on monitoring once they are conceived, and you can expect to be referred to an OB-GYN who specializes in high risk pregnancies.

Your doctor will usually screen for abnormalities like TTTS and TRAP sequence and ask that you continue frequent monitoring throughout the pregnancy.

Depending on a particular pregnancy’s risk factors, you may require in-patient care with continuous monitoring after a certain point in your pregnancy.

To protect complications like cord entanglement, mo/mo twins are frequently born via C-section and delivered early.

Whether you are a twin, are having twins, or just know twins, it’s fascinating to think about the processes involved in creating two humans at the same time!

For those expecting twins, understanding twin types can help to identify and address risk for issues caused by mo/mo pregnancies.

If you’re expecting twins it’s important to communicate regularly with a care provider to ensure the healthiest pregnancy.