Multiples What Are Monoamniotic-Monochorionic (MoMo) Twins? By Pamela Prindle Fierro Pamela Prindle Fierro Pamela Prindle Fierro is the author of several parenting books and the mother of twin girls. Learn about our editorial process Updated on July 24, 2020 Medically reviewed Verywell Family articles are reviewed by board-certified physicians and family healthcare professionals. Medical Reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Sarah Rahal, MD Medically reviewed by Sarah Rahal, MD LinkedIn Sarah Rahal, MD is a double board-certified adult and pediatric neurologist and headache medicine specialist. Learn about our Medical Review Board Print kumacore / Getty Images Table of Contents View All Table of Contents How MoMo Twins Form Diagnosis Risks Treatment The terms MoMo and mono-mono are short for "monoamniotic-monochorionic" and refer to twins who develop in a single amniotic sac and share a placenta. The words literally translate to mean 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 MoMo 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 MoMo twins. Only about 1% of twin pregnancies are monoamniotic. The majority of monozygotic twins will develop with separate sacs, or sometimes with separate amnions within a shared chorion, which are described as monochorionic-diamniotic or MoDi. MoMo babies always have identical features and are of the same sex because they derive from the same gene set. Diagnosis Most twin pregnancies are routinely monitored with ultrasound. Doctors will 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 MoMo twins. MoMo 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 MoMo multiples should be cared for by a perinatologist (an obstetrician specializing in high-risk pregnancies) or at least consult with a doctor experienced with MoMo twins. Monoamniotic-monochorionic twins face many potential health risks throughout pregnancy. Umbilical Cord Complications The twin fetuses connect to the shared placenta via their own umbilical cords, which supply blood and nutrients that help them grown 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 MoMo 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 MoMo 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 MoMo 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. Weight inequality can be a concern in MoMo twins as it can lead to growth restrictions in the womb, known as intrauterine growth restriction (IUGR). 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. MoMo 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 MoMo twins is about 75% to 80%. Many MoMo 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. Health Problems Faced By Preemies 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. There is no approved treatment or procedure to address the risks of MoMo twins. The only resolution is the delivery of the babies. This is why all MoMo babies are born prematurely. Doctors have to balance the risks of the babies' condition in the womb versus the consequences of prematurity. A C-section is mandated 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 by C-section at around 34 weeks (if they are not born earlier). Was this page helpful? Thanks for your feedback! Get diet and wellness tips to help your kids stay healthy and happy. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit 3 Sources Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Prefumo F, Fichera A, Pagani G, Marella D, Valcamonico A, Frusca T. The natural history of monoamniotic twin pregnancies: a case series and systematic review of the literature. Prenat Diagn. 2015;35(3):274-80. doi:10.1002/pd.4538 Cordero L, Franco A, Joy SD. Monochorionic monoamniotic twins: neonatal outcome. J Perinatol. 2006;26(3):170-5. doi:10.1038/sj.jp.7211457 Lee YM. Delivery of twins. Semin Perinatol. 2012;36(3):195-200. doi:10.1053/j.semperi.2012.02.004.