Monochorionic monoamniotic (mo/mo) twins are two identical babies from one fertilized egg, who develop with the same placenta and in the same amniotic sac.
Most people have heard of or know a set of identical or fraternal twins and may understand that identical twins come from one egg and fraternal twins come from two eggs. But the science of twins can get complex, and there are many other types of twins.
Here’s information about monochorionic monoamniotic, or mo/mo, twins. These identical twins share a placenta and an amniotic sac. This type of twin is extremely rare, occurring in only around 1 in 10,000 or 0.01% of all pregnancies.
Read to learn the terminology of twins, how mo/mo twins happen, and what makes them so rare.
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 in which there is one placenta and one amniotic sac for both babies.
Twin terminology
Confused about how this works? Here are some terms used to explain twins, how they form, and what happens in the womb.
Most people have heard mention of identical and fraternal twins:
- Identical twins (also called monozygotic twins): These 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): These twins come from two different eggs that are released close together and fertilized by two different sperm. They share only 50% of their chromosomes as other siblings do, and characteristics like hair and eye color or gender can be the same or different.
When it comes to identifying types of twins, classification depends on whether the fetuses shared or had separate chorionic and amniotic sacs. Here are more terms that explain this part of twin development:
- Amniotic sac: The sac that surrounds a fetus in the womb. It’s filled with amniotic fluid.
- Chorion: The outer layer of the amniotic sac.
- Amnion: The inner layer of the amniotic sac.
- Zygote: The first cell that forms after a sperm and egg merge. It has two sets of chromosomes – one set from the sperm and one set from the egg.
While developing in the womb, the chorion is the outermost fetal membrane, the outer layer of the amniotic sac. It connects 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 of their own.
Types of twins in the uterus

Even though two babies may share the same placenta (one chorion for the pair), they can each still be in their own amniotic fluid if they have their own amniotic sac (two separate amnions).
This means that there are actually 3 different twin combination possibilities:
- Di/di twins (dichorionic diamniotic): Each twin has their own individual chorion and individual amnion.
- Mo/di twins (monochorionic diamniotic): The twins share a chorion, but have separate amnion.
- Mo/mo twins (monochorionic monoamniotic): These twins have 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 25% to 30% 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 rare, accounting for only about
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 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 shows one placenta supplying blood to two fetuses. Both 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 (IUGR), which refers to the fact 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 high risk. 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, one twin may restrict 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 babies and may require monitoring, premature delivery, or surgical repair in utero.
- Twin reversed arterial perfusion sequence (TRAP sequence): Occurring in only about 2.6% of mo/mo pregnancies, one twin is developmentally typical (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 inpatient care with continuous monitoring after a certain point in your pregnancy.
To protect against 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!
Understanding twin types can help those expecting twins identify and address the risk of conditions 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.