Monoamniotic Twins

Monoamniotic twins are identical twins that share an amniotic sac, the fluid-filled sac that holds the baby during pregnancy (also known as the “bag of waters”). Normally, identical twins each have their own amniotic sac.

Monoamniotic twins are a rare type of “monochorionic” twins, meaning they also share a placenta. They are sometimes referred to as monochorionic-monoamniotic twins, or monoamniotic for short.

Twins that share both an amniotic sac and a placenta face increased risks compared to other twin pregnancies, with the potential for severe complications – including umbilical cord entanglement or compression – that can threaten the survival of one or both babies.

These uncommon pregnancies are estimated to occur in only 1 percent of identical twin pregnancies.

Diagnosis

Early diagnosis of monoamniotic twins is vital to recognizing the unique risks associated with these pregnancies, ensuring close monitoring for early detection of complications.

The diagnosis is typically made during a routine prenatal ultrasound in the first trimester that shows a single amniotic sac containing two fetuses. In some cases the ultrasound may show the twins’ umbilical cords are entangled, an indication the fetuses share a single amniotic sac. In these cases, the imaging should also detect a shared placenta.

A misdiagnosis of monoamniotic twins sometimes occurs early in a twin pregnancy because the membrane that separates two amniotic sacs is so thin it may not be visible, giving the appearance of a single sac. A follow-up ultrasound that detects the dividing membrane will reveal the twins are in two separate sacs, changing the diagnosis to “diamniotic” twins (two amniotic sacs). Transvaginal ultrasonography may be needed to confirm the diagnosis of a monoamniotic twin pregnancy.

Potential Complications

In addition to the risks common to all twin pregnancies, such as preterm birth, monoamniotic twins are at increased risk of serious complications caused by both the shared amniotic sac and the shared placenta. The shared amniotic sac can result in the twins’ umbilical cords becoming entangled, with severe consequences, while the shared placenta allows the fetuses to share a blood supply, placing the twins at risk of uneven blood flow and uneven blood volume that can threaten their growth and survival.

Potential complications include:

• Umbilical cord entanglement or compression, which can restrict or cut off the blood supply to the fetuses leading to death of one or both twins
• Preterm birth, a leading cause of infant death and childhood disability
• Selective fetal growth restriction (sFGR) - poor growth and development in one twin caused by unequal sharing of the placenta
• Twin-twin transfusion syndrome (TTTS) - a relatively rapid transfer of blood from one fetus to the other that, if untreated can lead to fetal heart failure and death of one or both twins
• Twin anemia polycythemia sequence (TAPS) - a relatively slow transfer of fetal red blood cells from one fetus to the other, that if untreated can threaten the survival of one or both babies
• Twin reversed arterial perfusion (TRAP) sequence - abnormal blood flow patterns that result in one healthy twin and one severely malformed (acardiac) twin
• Amniotic fluid abnormalities, typically polyhydramnios (too much fluid), which may be an early sign of TTTS in monoamniotic twin pregnancies
• Congenital malformations, including heart defects, neural tube defects, and brain abnormalities
• Developmental delays
• Intrauterine death of one or both twins
• Conjoined twins
• Preeclampsia - maternal high blood pressure with signs of organ damage
• Postpartum hemorrhage

Specialized Evaluation and Prenatal Care

If monoamniotic twins are diagnosed, you may be referred to a fetal center for further evaluation.

At Texas Children’s Fetal Center, we arrange for you to visit as quickly as possible for a comprehensive assessment by a team of specialists experienced in the diagnosis and treatment of these rare and complex twin pregnancies, including maternal-fetal medicine physicians (OB/GYNs specializing in high-risk pregnancies), fetal surgeons, fetal imaging experts, fetal cardiologists, and neonatologists.

Additional testing will be performed using state-of-the-art imaging technologies and techniques that provide detailed views of your twins’ condition, for the most accurate diagnosis and treatment planning. Testing may include:

• High-resolution anatomy ultrasound to confirm the diagnosis and screen for abnormalities such as twin-twin transfusion syndrome (TTTS) or other signs of complications
• Ultra-fast MRI for a more detailed view of fetal anatomy
• Fetal echocardiogram to evaluate the babies’ hearts
• Doppler ultrasonography to examine blood flow between the twins
• Amniocentesis and chromosomal analysis to identify any chromosomal anomalies

Our specialists will then meet with you about the results, discuss treatment recommendations and answer any questions your family has, to help you make the most informed decisions regarding your twins’ care and treatment.

Treatment During Pregnancy

Mother and babies will be closely monitored throughout pregnancy with frequent ultrasounds and fetal echocardiograms to assess fetal growth, fetal heart function, umbilical cord entanglement, and amniotic fluid levels, among other factors, for early detection of complications.

Treatment during pregnancy depends on the health of the mother and babies. In general, treatment strategies in monoamniotic twin pregnancies may include:

• Steroids to accelerate fetal lung development, in case of early delivery
• Laser surgery if you develop advanced twin-twin transfusion syndrome
• Radiofrequency ablation, umbilical cord occlusion or laser surgery in you develop TRAP sequence
• Amnioreduction, a procedure used to reduce the amniotic fluid level, decreasing the risk of preterm labor
• Medications to stop preterm labor contractions
• Hospitalization in some cases to monitor the twins more closely

If complications occur during these pregnancies, they are often complex conditions that require accurate diagnosis and specialized care to ensure proper treatment. If fetal intervention is recommended, talk with your healthcare team about the potential benefits and risks, to help you make the best decision for your family.

Delivery

Delivery should take place at a center with the expertise and resources required to treat these rare twin pregnancies and deliveries, including the highest level neonatal intensive care unit (NICU) for premature and critically ill newborns. 

Preterm delivery is common in monoamniotic pregnancies. A cesarean delivery may be necessary if umbilical cord complications occur.

Delivery timing and postnatal care should be carefully planned and coordinated, balancing the risk of complications to the twins in utero with the risks associated with preterm birth. Our Fetal Center team works closely with pediatric experts from Texas Children’s Hospital, consistently ranked one of the best children’s hospitals in the nation by U.S. News & World Report.

Treatment After Birth

Treatment after birth depends on the health of the newborns. Monoamniotic twins often require a stay in the NICU for breathing support, feeding assistance, or other needs associated with prematurity, or for treatment of complications that occurred during pregnancy.

Additional treatment may be required for any congenital defects present.

Depending on your babies’ condition, their postnatal care team may include:

• Neonatologist
• Pediatric cardiologist
• Primary pediatrician
• Pediatric surgeon

Preterm birth is a leading contributor to childhood disability, as many of the baby’s vital organs – including the lungs, brain, and heart – are still developing. Monoamniotic twins may require long-term care and follow-up due to complications associated with preterm birth.

Why Texas Children’s Fetal Center?

• A single location for expert maternal, fetal, and pediatric care. At Texas Children’s Hospital, mother and babies receive the specialized care required for the diagnosis and treatment of monoamniotic twin pregnancies and complications all in one location, including immediate access to our level IV NICU, if needed.
• A skilled, experienced team with proven outcomes. We have a dedicated team of maternal-fetal medicine (MFM) specialists, fetal surgeons, fetal imaging experts, neonatologists, pediatric cardiologists, and others who work in concert to care for you and your babies every step of the way, using proven protocols we’ve developed over the years. With their combined expertise and unified approach, this team offers the best possible care for pregnancies involving monoamniotic twins.
• We care for your children’s needs at every stage of life. Our comprehensive approach starts with your first prenatal visit and continues through delivery, postnatal care, and childhood, as needed, thanks to one of the nation’s leading teams of fetal and pediatric specialists for the care and treatment of fetal conditions and abnormalities.

Videos

• Texas Children’s Fetal Center: Our care began before he/she was born

Additional Resources

• 6 things to consider when choosing a fetal center
• The Women’s Place