What Are Mono Mono Twins?

Mono mono twins are identical twins who develop in a single amniotic sac and share a placenta. The terms mono-mono and MoMo are short for "monoamniotic-monochorionic." This refers to a single chorion (outer membrane surrounding an embryo) and a single amniotic sac (the bag of waters that contains the fetuses). This situation is very rare and may cause risk to the babies due to cord entanglement and other issues.

How Mono Mono Twins Form

Identical (monozygotic) twins develop from a single egg-sperm combination that splits into two. If the split is delayed, usually a week or so after conception, the processes of growing a placenta, chorion, and amniotic sac are already underway. The two embryos will then develop within a single, shared sac, resulting in mono mono twins.

The majority of monozygotic twins develop with separate sacs, or sometimes with separate amnions within a shared chorion (described as monochorionic-diamniotic or MoDi). MoMo babies are always identical twins and are of the same sex because they derive from the same gene set.

Diagnosis

Most twin pregnancies are routinely monitored with ultrasound. Doctors look for the presence of a dividing membrane that indicates that the two fetuses are in separate sacs.

The lack of a membrane, or a thin or vague line, may prompt further analysis to confirm the situation. Ultrasound is the only way to detect mono mono twins.

Mono mono twins are often misdiagnosed in the early weeks of pregnancy when the membrane is so thin as to be nearly invisible. Often, a later ultrasound reveals a dividing membrane, confirming that twins are actually monochorionic-diamniotic.

Risks

Pregnant parents of mono mono twins should be cared for by a perinatologist (an obstetrician specializing in high-risk pregnancies) or at least consult with a doctor experienced with mono mono twins. Monoamniotic-monochorionic twins face many potential health risks throughout pregnancy; their survival rate is 70%.

Umbilical Cord Complications

The twin fetuses connect to the shared placenta via their own umbilical cords, which supply blood and nutrients that help them grow and develop. As the babies move around in the same amniotic sac in the uterus, the cords can cross or press against each other, cutting off these vital lifelines.

This can be a life-threatening situation. The longer the cords are entwined or compressed, the greater the risk of damage to the cords—and the greater the risk of death for one or both babies.

Twin-to-Twin Transfusion Syndrome

Mono mono twins are susceptible to twin-to-twin transfusion syndrome (TTTS), which happens when one twin (the donor) essentially provides a blood transfusion to another twin (the recipient). The recipient twin often receives the majority of the nourishment in the womb, leaving the donor twin undernourished (smaller and often anemic).

A doctor can diagnose TTTS in any set of twins by examining fluid levels in their amniotic sacs. However, the fact that mono mono twins only have one sac makes a TTTS diagnosis much more difficult. Comparing the physical development of both of the twins is the only way to diagnose this condition prior to birth.

Abnormal Amniotic Fluid Levels

Mono mono twins can be affected by amniotic fluid levels that are either too low (oligohydramnios) or too high (polyhydramnios). Low blood supply in one of the twins will lead to not enough amniotic fluid.

This limits movement, bladder size, and overall fetal growth, in addition to decreasing the protection from compression of the umbilical cord in the uterus. A larger than normal blood supply will result in excess amniotic fluid, leading to an enlarged bladder and the possibility of heart failure. 

Twin Reversed Arterial Perfusion Sequence

Monochorionic twins are at greater risk of twin reversed arterial perfusion sequence (TRAP sequence). In this condition, one twin's heart (and sometimes other parts of the body as well) fails to develop and the other twin's heart works for both babies.

The twin without a heart cannot survive, and the other twin can experience heart failure because its heart is working so hard. Treatment involves either early delivery or interrupting the blood supply between the twins so that the healthier twin has a greater chance of survival.

Low Birth Weight

Low birth weight is independently linked to reduced odds of survival and a higher risk for disabilities and health problems in life. Mono mono twins have four times the risk of low birth weight as compared to pregnancies in which each fetus has a placenta of its own.

Preterm Birth

After 24 weeks, the survival rate of mono mono twins is about 75% to 80%. Many mono mono twins experience life-threatening complications as early as 26 weeks, resulting in spontaneous preterm delivery or an earlier than planned Cesarean section (C-section). Preterm delivery is often associated with a number of other life-threatening conditions.

Monitoring and Treatment

Fortunately, modern technology allows doctors to observe babies in the womb and monitor the situation. High-resolution ultrasounds, Doppler imaging, and non-stress tests help to assess symptoms and identify potential cord problems.

Cord entanglement and compression are usually slow processes, so parents and medical caregivers have time to make decisions. Some situations will require close monitoring of the pregnancy in the hospital. Sometimes steroids may be administered to boost the babies' lung development and improve their chances of surviving outside the womb.

Doctors have to balance the risks of the babies' condition in the womb versus the consequences of prematurity. A C-section is usually recommended for MoMo babies to avoid cord prolapse, a situation that occurs when the second baby's cord is expelled as the first baby is delivered.

If cord compression occurs early in the pregnancy, the babies may not be able to survive. The risk of cord entanglement and compression is simply too great after 34 weeks, so all MoMo twins are delivered at around 34 weeks (if they are not born earlier). One small study found that vaginal delivery of mono mono twins was safe, but it involved only 29 births.

A Word From Verywell

Learning that your babies are mono mono twins can be frightening due to the extra risks they face. Careful monitoring will be important, and so will a support system that you can count on to help manage stress and anxiety during this high-risk pregnancy.