Birth defects and anomalies in monochorionic twins

A birth defect or anomaly is a medical term meaning irregular or different from normal. Anomalies occur more frequently in identical twins than in other pregnancies.

What are the different kinds of twins?

  • Monochorionic twins are identical twins who share one placenta. This occurs in approximately 70 percent of pregnancies with identical twins. 
  • Monochorionic-monoamniotic twins are identical twins who share both a placenta and an amniotic sac. 
  • Monochorionic-diamniotic twins are identical twins who share a placenta but not an amniotic sac. 
  • Dichorionic twins each have their own placenta and amniotic sac. This occurs in approximately 30 percent of pregnancies of identical twins. All fraternal (non-identical) twins also are dichorionic twins.

Problems that can affect the health of monochorionic twins include: 

Prenatal diagnosis of monochorionic twin anomalies

Doctors typically detect monochorionic twin pregnancies during a routine prenatal ultrasound. The best images are visible when ultrasounds are conducted prior to 14 weeks of pregnancy. This is when the structures of the placenta and amniotic sac are most visible. The ultrasound will show a single placenta supplying blood flow and two fetuses with a single, thin dividing membrane if they are monochorionic twins.

At this point, the ultrasound may also show if there is one amniotic sac or two. If there are two, physicians will look to see if there are differences in the amount of amniotic fluid present in each. 

What happens after diagnosis?

Depending on the ultrasound findings, your obstetrician will likely refer you to a maternal-fetal medicine specialist (a doctor who specializes in high-risk pregnancies). Additional maternal and fetal tests may be required to screen for anomalies, such as TTTS and TRAP sequence. One screening test is a fetal echocardiography. This specialized ultrasound of your babies’ hearts and blood vessels shows the flow of blood between the twins.

To monitor the size of your babies, along with their blood supply and amniotic fluid, your doctor will order repeat testing and ultrasounds throughout your pregnancy.

How do monochorionic anomalies affect my babies?

Twins in general are more likely than single fetuses to face challenges with development and delivery. These chances increase for monochorionic twins. Some concerns are:

1. Low birth weight

Birth weight is a factor for nearly all monochorionic twins. All twins are 10 times more likely to have a lower birth weight than babies from single pregnancies are. For monochorionic twins, the risk of low birth weight is four times higher than in pregnancies in which each fetus has its own placenta. 

2. Weight inequality between your twins

Weight inequality also can be a concern. A significant difference in the weight of the babies is more common among monochorionic twins. When there is not enough nutrition for one or both fetuses to develop at a normal rate, intrauterine growth restriction (IUGR) can occur. 

3. Differing amniotic fluid levels

Unequal amniotic fluid levels can affect your twins. Twins deprived of a blood supply will produce too little amniotic fluid – a condition called oligohydramnios. This condition limits their movement and bladder size, among other consequences. A larger than normal blood supply will result in excess amniotic fluid (polyhydramnios), enlarged bladder and the possibility of hydrops (heart failure). 

4. Cord entanglement or compression

Cord entanglement or compression is a risk for all monochorionic-monoamniotic twins who share the same space in a single amniotic sac. Umbilical cords are the lifeline to blood and nutrients for all fetuses. Like a garden hose, if the cords flatten or bend, the supply line can be damaged or cut off, impacting fetal development and increasing the risk of death. 

5. TTTS and TRAP sequence

Two conditions called TTTS and TRAP sequence may develop in rare circumstances. These conditions, caused by inequalities in the blood supply, have serious implications for the fetuses and require highly specialized care. If TTTS is not detected and treated, it is likely to result in the loss of both babies. In TRAP sequence, one twin will not survive because it does not have a developed heart and brain structure. The survival rate for the healthy twin is 25 to 50 percent when TRAP sequence is not detected and treated. 

How do anomalies in monochorionic twins affect the pregnancy?

Care guidelines for monochorionic pregnancies with anomalies are similar to those for mothers carrying all twins. Depending on your babies’ needs, your doctor will make adjustments in your care plan. For example, your doctor may change guidelines for:

  • Nutrition
  • Rest
  • Prenatal visits
  • Other factors

Your doctor will also want to closely observe your babies and monitor their health through ultrasound and other tests. 

Your doctor may also recommend:

  • Bed rest
  • Tocolytic medications (medications to stop contractions or labor)
  • Other measures to prevent early delivery

There is a possibility you will have to be hospitalized for constant monitoring. This is the case for babies with cord entanglement, TTTS and TRAP sequence, for example.

What to expect during labor and delivery of monochorionic twins

There is a good chance your labor and delivery will be early. Premature delivery is the most common complication of twin pregnancy. This rate does not increase significantly when the pregnancy is monochorionic, but the rate of very early delivery (before 32 weeks) is nearly twice as high when compared with dichorionic twins. Even without major complications, most doctors will recommend that monochorionic twins be delivered at least three weeks before the official due date. 

There is also a greater incidence of cesarean section deliveries in monochorionic pregnancies, especially when they are monoamniotic and at risk for cord entanglement at birth.

How are abnormalities and complications in monochorionic twins treated?

Treatments for complications of monochromic pregnancies vary depending on the condition of the babies. 

Cord entanglement

In the case of cord entanglement, physicians may recommend delivery of the twins as soon they are mature enough to survive, which may be as early as 24 weeks. In these cases, your doctor may give you corticosteroid medications to accelerate lung development in your twins so they have a greater chance of surviving outside of the womb. Many cases of cord entanglement need extensive monitoring of the twins in the hospital to help determine the best time to deliver them.

Amniotic fluid issues

If excessive amniotic fluid is a concern, your physician may perform an amnioreduction to reduce the amount of amniotic fluid. This procedure will:

  • Make you more comfortable
  • Reduce the chance of premature labor
  • Better equalize the flow of blood between your twins

Your physician may need to repeat this procedure throughout your pregnancy. 


In the case of TTTS, an increasingly successful approach involves disconnecting part of the blood supply between the twins using a technique called fetoscopic laser ablation of the shared placental vessels, otherwise known as laser ablation.

This procedure is only offered at select treatment centers, like Children’s Wisconsin through the Fetal Concerns Center. In 80 to 85 percent of laser surgery procedures, at least one twin survives; both twins survive in 65 to 70 percent of cases. The survival rate of both twins with TTTS without laser ablation is 10 to 20 percent. 

TRAP sequence

If your doctor has diagnosed TRAP sequence, he or she may recommend delivery of the healthy twin if your pregnancy is far enough along. In other cases, your doctor may recommend prenatal treatments to stop the extra pumping between the twins. These procedures, performed using highly specialized techniques within the womb, may include radiofrequency ablation. RFA is usually performed between 17 and 24 weeks of pregnancy. 

RFA is a procedure performed by our team of experts in the Fetal Concerns Center and at a very limited number of neonatal centers across the country. The procedure uses heat energy to stop the blood flow to the umbilical cord of the underdeveloped twin. RFA has been shown to be extremely successful, with a 90 percent or higher survival rate for the normal twin when delivered at least 35 weeks into the pregnancy. 

Are there risks of treatment?

Procedures such as amnioreduction, laser ablation or RFA, like any surgeries, pose a risk of bleeding and infection that can lead to complications for both the mother and baby. 

Other potential complications related to fetal surgery include injury to the baby and premature labor and delivery. 

If you have one of these procedures, you will most likely be admitted to the hospital for monitoring for two to three days. After your stay, your doctor will discharge you for ongoing care by your obstetrician. You may have to be on bed rest and you may have to take medications to prevent premature delivery. 

If your babies are born prematurely or with special needs, they will be cared for in a special care nursery called a neonatal intensive care unit. At the NICU, expert physicians and staff can quickly respond if your babies need specialized care.

Will I be able to help care for my babies?

Yes. Ask your nurse about ways to interact with and care for your babies. Babies that are healthy at birth may need only a brief stay in a special care nursery. Others may need more time in the NICU to resolve health issues. 

Breastfeeding and pumping/storing

Many mothers are able to successfully breastfeed their twins. Lactation specialists can help mothers of multiples learn techniques for breastfeeding their babies separately and together and increase their milk supply. 

Mothers whose babies are unable to breastfeed because they are sick or premature can pump their breast milk and store the milk for later feedings.

Getting help with your babies

Families with more than one baby need help from family and friends, especially if the pregnancy was complicated and there are other siblings at home. Having help will allow you more time to get to know your babies, take care of feedings and rest and recover from delivery.

When can my babies go home?

Most monochorionic twins with anomalies will need to be cared for in the NICU. How long your babies remain hospitalized will vary depending on their conditions. Once babies are able to feed, grow and stay warm, they usually can go home.

What is the long-term prognosis for my babies?

The long-term prognosis depends on the types of complications caused by the monochorionic pregnancy, including birth weight and prematurity. Low-birth-weight babies have an increased risk of long-term problems, such as: 

Babies born between 24 and 32 weeks have a high risk of death or severe handicaps if they survive. Babies born after 32 weeks often face feeding and breathing problems, but they nearly always survive. 

What about future pregnancies?

Having a previous monochorionic pregnancy does not influence whether or not you will have another one. The incidence of identical twins is not influenced by family history, so the chances of having them are the same for all women – approximately 1 in 285 pregnancies. 

The chance for any woman to have a monochorionic pregnancy with anomalies is approximately 1 in 5,816 pregnancies or less than 0.05 percent. 

Contact us

For additional information on the Fetal Concerns Center at Children's Wisconsin, please call:

(414) 337-4776

Fax: (414) 337-1884

Note: These phone numbers should not be used for urgent medical concerns. Please contact your physician directly if your situation requires immediate attention, or dial 911 if it is an emergency.