Monochorionic Twins

Monochorionic twins are genetically identical twins that share a placenta. The placenta is the organ that develops in the uterus during pregnancy and attaches to the uterine wall, providing nourishment and oxygen to the growing babies via their umbilical cords. 

While all twins are at increased risk of complications compared to a “singleton” pregnancy (one baby), monochorionic twins face even greater risks as a result of the shared placenta. In some cases complications can be severe, threatening the life of one or both babies. 

An estimated 70% of identical twins are monochorionic. In the remaining cases, each twin has their own placenta (known as “dichorionic” twins).

Two Types of Monochorionic Twins

In some cases, monochorionic twins may also share an amniotic sac, the fluid-filled sac that holds the baby (sometimes referred to as the “bag of waters”).

As a result, there are two types of monochorionic twins:

• Monochorionic-diamniotic twins are identical twins that share a placenta but each develops in their own separate amniotic sac. This is the most common type.
• Monochorionic-monoamniotic twins are identical twins that share both a placenta and an amniotic sac. This is the rarest and highest risk form of twin pregnancies.

Diagnosis

Early diagnosis of monochorionic twins is important for optimizing outcomes in these high-risk pregnancies, ensuring close monitoring for early detection of complications.

The diagnosis can be made during a routine prenatal ultrasound in the first trimester that shows two fetuses and a single, shared placenta. The determination of whether both twins share the placenta or not is very important because the risks of pregnancy complications are much higher when both babies share the placenta. The best timing to make this determination is the first trimester of pregnancy. The number of amniotic sacs can also be detected.

In some cases, the imaging at the time of diagnosis may also identify early complications caused by the shared placenta.

Potential Complications

In addition to the risks common to all twin pregnancies, such as preterm birth, monochorionic twins are at increased risk of often severe complications caused by the shared placenta.

Blood vessel connections within the shared placenta allow the fetuses to share a blood supply. This can lead to uneven blood flow and uneven blood volume that can threaten the growth and survival of one or both twins.

Potential complications include:

• Preterm birth and low birth weight
• 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
• Umbilical cord complications, including entanglement or compression, which can restrict or cut off the blood supply to the fetuses. This is one of the risks of twins that share the amniotic sacs (monoamniotic twins).
• Amniotic fluid abnormalities, either oligohydramnios (too little fluid) or polyhydramnios (too much fluid)
• Congenital malformations, including heart defects, neural tube defects and brain abnormalities
• Developmental delays, which can range from minor learning difficulties to several palsy
• Intrauterine death of one or both twins
• Preeclampsia (maternal high blood pressure with signs of organ damage)
• Postpartum hemorrhage

Specialized Evaluation and Prenatal Care

If monochorionic 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 evaluation and treatment of these 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 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 that you and your family have, 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, 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 monochorionic twin pregnancies may include:

• Laser surgery if you develop advanced twin-twin transfusion syndrome and in selected TAPS cases
• Radiofrequency ablation or laser surgery in you develop TRAP sequence
• Intrauterine transfusion when there is suspicion that either twin has severe anemia
• Steroids to accelerate fetal lung development, in case of early delivery
• Amnioreduction, a procedure used to reduce the amniotic fluid level, decreasing the risk of preterm labor
• Medications to stop preterm labor contractions
• Hospitalization 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 monochorionic twin pregnancies and deliveries, including the highest level neonatal intensive care unit (NICU) for premature and critically ill newborns. 

Monochorionic pregnancies are at high risk of preterm delivery. In some cases a cesarean delivery may be necessary, for example when cord complications occur.

Delivery and postnatal care should be carefully planned and coordinated, balancing the risk of complications to the twins in utero with the risks to the babies 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. Monochorionic twins may require a stay in the neonatal intensive care unit (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. Monochorionic 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 monochorionic 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 with monochorionic 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