Guide to Mo/Mo Twins: Understanding Development and Risks

| Reviewed By Amanda Lundberg, BSN, RN

If you find out that you are pregnant with twins, the next course of action is to identify the type of twins you are expecting. This knowledge will offer valuable understanding for you and your physician regarding the possible risks to your pregnancy.

What are mo/mo twins? Mo/mo twins are monochorionic/monoamniotic twins, and they are rare. Mo/mo twins are identical twins who come from the same fertilized egg. However, the egg does not split until nine days after fertilization, and that means the twins share one placenta and one amniotic sac.

There are risks when carrying mo/mo twins, and your doctor will monitor the pregnancy closely.

Monochorionic Monoamniotic Twins

Though they make up a very small percentage of identical twins, we now know quite a bit about mo/mo twins.

How Twins Develop – The Basics

Identical twins develop when one fertilized egg splits. When it splits will determine if the twins are mo/mo, mo/di, or di/di.

Mo/mo twins split after the ninth day of fertilization, and this makes it impossible for them to each have their own amniotic sac.

Understanding Mo/Mo Twins

When a fertilized egg doesn’t split until after the ninth day of fertilization, you end up with mo/mo twins. Splitting that occurs this late means each twin will have to share a chorionic and amniotic sac.

The chorionic sac helps form the placenta, so mo/mo twins will have to share one. The amniotic sac is full of amniotic fluid and protects a fetus in utero.

Mo/mo twins have to share an amniotic sac and the amniotic fluid, so they are in the same space without a membrane to separate them.

How Rare Are Mono Mono Twins?

Your chances of having mono mono twins is very low. Out of all identical twin pregnancies, only one percent are mono mono.

Are All Mo/Mo Twins Identical?

All mo/mo twins are identical since they come from the same fertilized egg. 

Monoamniotic Twins Gender

Mono mono twins are always the same gender. Most mono mono twins are girls, with over 55 percent being female.

Why Are Mo/Mo Twins Dangerous?

A pregnancy with mo/mo twins is dangerous because the twins are sharing an amniotic sac. While it’s risky for twins to share a placenta, this occurs in mo/di pregnancies and usually is successful with close monitoring.

However, sharing an amniotic sac is different.

When twins share an amniotic sac, their umbilical cords can get knotted. There isn’t a membrane separating them, so as they move in the womb, the cords can tangle, clamp, or knot, resulting in death.

That’s why monitoring this type of pregnancy is extremely important.

Mo/Mo Twins Risks

While it is understandably scary to contemplate what could go wrong with your pregnancy, it is nonetheless important to be well informed.

Cord Entanglement

When twins share an amniotic sac, their umbilical cords can get tangled, knotted, or clamped. This means one or both twins will not receive what they need to survive.

If this happens before your twins can be delivered by cesarean section, one or both will not survive.

Twin-to-Twin Transfusion Syndrome

Twin-to-twin transfusion syndrome occurs when one twin offers blood to the other twin, leaving the first without enough. If this is caught in time, doctors can try to intervene.

However, it’s harder to detect TTTS in mo/mo twins because they are sharing an amniotic sac.

Monitoring amniotic fluid levels is how TTTS is diagnosed in many pregnancies, so doctors have to use other methods with mo/mo pregnancies and can miss TTTS altogether.

Intrauterine Growth Restriction

All twins run out of space faster than singletons. Mo/mo twins are no exception, and they are in each other’s space more than mo/di or di/di twins because they are sharing everything.

If they run out of room, they may be growth restricted and suffer from low birth weight.

Mo/Mo Twins Survival Rate

The mo/mo twin survival rate is currently around 70 percent.

Mo/Mo Twins Miscarriage

While there are no definitive stats when it comes to mo/mo twin miscarriages, twin pregnancies are more likely to end in miscarriage. One or both twins may be lost.

How Do You Know if Your Twins Are Mo/Mo?

An ultrasound can tell you more about what type of twins you’re having, but you may need to see an MFM doctor to diagnose mo/mo twins for sure.

The thin membrane separating mo/di twins is not always easy to see, so you may mistakenly be told your twins are mo/mo when they are not.

A high-risk pregnancy doctor will have the type of ultrasound you need to tell if there really isn’t a membrane, making your twins mo/mo.

Monochorionic Monoamniotic Twins Delivery

Mo/mo twin pregnancies will not go to full term. The risks to the babies are too high after 34 weeks, and many have to be delivered before then. Most doctors recommend in-hospital monitoring of your twins once you reach 28 weeks until you give birth.

At this point, your twins have a chance of surviving outside of the womb if their cords become tangled or twin-to-twin transfusion syndrome puts them at risk.

From 28 weeks on, you will be monitored in the hospital, and your babies will be delivered via C-section if there is a problem or at 34 weeks, whichever comes first.

Types of Twin Pregnancy

There are different types of twin pregnancies, and each one has its own features.

Di/Di Twins

Di/di twins graphic

Di/di twins have their own chorionic and amniotic sacs. Most di/di twins are fraternal, but a small percentage are identical.

Mo/Di Twins

Mo/di twins graphic

Mo/di twins share a placenta but do not share an amniotic sac. This type of pregnancy is still considered high risk, but both twins having their own amniotic sacs makes it less risky than a mo/mo pregnancy.

Mo/Mo Twins

Mo/mo twins graphic

Mo/mo twins share a placenta and an amniotic sac. Mo/mo twins are rare, and these pregnancies come with great risks.

Monozygotic Twins vs. Dizygotic Twins

Monozygotic twins form when one fertilized egg splits. They are identical twins.

Dizygotic twins come from two eggs fertilized by different sperm. This creates fraternal twins.

How Do Fraternal Twins Form?

Fraternal twins are formed when two eggs are released and fertilized by two separate sperm. They only share about 50 percent of their DNA, so they are genetically as similar as any other non-twin siblings.

How Do Identical Twins Form?

Identical twins form when one fertilized egg splits. When the egg splits will determine what type of identical twins you’re having and how much they are sharing in the womb.

Related Questions: 

Do All Identical Twins Look Alike?

Since identical twins share the majority of their DNA, they do tend to look very much alike. However, genetic mutations in the womb can leave differences in appearance, though they may be slight.

Do Identical Twins Have To Be The Same Sex?

Since identical twins come from the same fertilized egg, they will be the same sex.

Final Thoughts

A mo/mo twin pregnancy is risky, but current technology makes it easier to monitor twins in the womb and intercede when possible.

Take good care of yourself, work closely with your doctor, and focus on positive thinking until those little babies are safely in your arms.