What Are Monoaminotic Twins?

When your baby is in the womb, they have an amniotic sac to provide protection as they grow. They also have a placenta to provide blood flow and nutrients during pregnancy. Even when you’re pregnant with twins, most of the time they each have their own sac and own placenta. However, some identical twins share both an amniotic sac and a placenta, making them monoamniotic twins.

Understanding Monoaminotic 'Momo' Twins

Monoamniotic twins may also be referred to as monochorionic, monochorionic-monoamniotic, or momo twins. This condition increases the risk of complications for both of your babies. Only around 1% of twin pregnancies result in monoamniotic twins.‌

Different types of twin pregnancies besides monoamniotic include: 

  • Monochorionic twins are identical twins who share a single placenta. They represent around 70% of identical twin pregnancies.  
  • Monochorionic-diamniotic twins are identical twins who share a placenta, but each has their own amniotic sac. ‌
  • Dichorionic twins each have their own placenta and amniotic sac. They represent around 30% of identical twin pregnancies. Fraternal twins are also considered dichorionic.

Health Risks of Monoaminotic Twins

Your risks already increase when you are pregnant with more than one baby. With less room to grow, twin pregnancies usually result in preterm birth. However, monoamniotic twins face additional risks because they share a single placenta and amniotic sac. These health risks include: 

  • Entangled umbilical cords that may become wrapped around your babies 
  • Compressed umbilical cords that end up in knots and cut off blood supply
  • Reduced blood supply for each baby because of sharing
  • Uneven blood flow that stunts growth in the womb, often affecting only one baby and also called selective fetal growth restriction (sFGR) 
  • Twin-to-twin transfusion syndrome (TTTS), the rapid transfer of blood from one baby to the other that can lead to heart failure and death of one or both twins 
  • Amniotic fluid abnormalities, meaning your babies may have too much or too little fluid
  • Congenital malformations, including defects of the heart, neural tubes, or brain
  • Delays in meeting expected milestones following birth‌
  • Twins who are joined together requiring separation after being born‌‌

You are also at risk as a mother carrying monoamniotic twins. Preeclampsia is common and may lead to organ damage if left untreated. Following the delivery of your babies, hemorrhage is also a possibility your doctor will be ready to treat.

Continued

Finding Out You're Having Monoaminotic Twins

It is important to know about momo twins early on in your pregnancy so you and your babies may be closely monitored for complications. When you’re pregnant with twins, you usually receive more ultrasounds than you would with a single-birth pregnancy because of increased risks.

Monoamniotic twins are usually identified during a first-trimester ultrasound. Once your babies are big enough, the ultrasound shows more detail. Identifiable signs of monoamniotic twins include:

  • Entangled cords
  • A single amniotic sac
  • Presence of only one placenta
  • Significantly more or less amniotic fluid‌

It is more difficult to identify separate amniotic sacs because the membranes are so thin that they may not be obvious on ultrasound imaging. Your doctor may request a follow-up ultrasound to see if a dividing membrane is distinguishable before making a final call on whether you are pregnant with monoamniotic twins.‌

If an abdominal ultrasound isn’t accurate enough, your doctor may request a transvaginal ultrasound. When the imaging is completed vaginally, it often provides a clearer picture and more accurate results.

Care for Monoaminotic Twins

Your doctor monitors you very closely once your pregnancy is diagnosed as monoamniotic. Your doctor wants regular readings on:

  • Fetal growth
  • Fetal heart rate and function
  • Umbilical cord tangles and knots‌
  • Amniotic fluid levels‌

Your doctor has charts for average fetal growth and readings and compares your babies’ results with what is expected. They can also see how your baby is progressing week after week compared with previous growth and rates.

Early detection is the key to keeping your babies safely in utero for as long as possible. It also helps your doctor to know when it may be time to induce your labor or schedule a cesarean section for your babies to be born sooner.

Your doctor may use specialized techniques to improve the likelihood of a full-term pregnancy and better outcomes for you and your babies, including:

Steroids: recommended to speed up your babies’ lung development in case an early delivery is necessary 

  • Laser surgery: used if your babies suffer from advanced twin-twin transfusion syndrome
  • Amnioreduction: a procedure that safely reduces the amount of amniotic fluid in your womb if levels are too high
  • Medication: meant to cease contractions that are strong enough to put you into preterm labor
  • Hospitalization: for high-risk situations in which you and your babies need to be monitored more closely‌‌

When it comes to a complex monoamniotic pregnancy, there are pros and cons to all decisions. Talk to your doctor about the risks and benefits of staying pregnant for longer versus delivering your babies sooner. Together you can make the best decision for the health of you and your babies.

WebMD Medical Reference Reviewed by Dan Brennan, MD on June 05, 2021

Sources

SOURCES:

Children’s Wisconsin: “Birth defects and anomalies in monochorionic twins.” 

Columbia University Department of Obstetrics and Gynecology: “Monoamniotic Twins.”

Texas Children’s Hospital: “Monoamniotic Twins.”‌

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