Birth Defects in Monochorionic Twin

What are the different kinds of twins?
Monochorionic twins are identical twins who share one placenta, which occurs in approximately 70 percent of pregnancies of 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 are identical twins who each have their own placenta and their own amniotic sac, and this occurs in approximately 30 percent of pregnancies of identical twins.

All fraternal or nonidentical twins will also have two placentas and two amniotic sacs.

A birth defect or anomaly is a medical term meaning irregularity or different from normal. Anomalies occur more frequently in identical twin than in other pregnancies. Other problems that affect the health of monochorionic twins include:

See the sections below for more information about these anomalies.

Anomalies other than birth defects in monochorionic twins are primarily related to two factors - blood supply and the amniotic sac.

Twins who share a placenta also share a blood supply. For this reason, although identical, monochorionic twins may grow to be different sizes. This is because the umbilical cords of monochorionic twins may implant on different places on the placenta resulting in unequal access to blood flow and nutrition.

One twin's cord may implant in a position that will provide a larger amount of the placenta and blood flow to one of the twins. Unequal blood flow can lead to differences in amniotic fluid in the amniotic sac. It is also possible that the umbilical cords may connect within the walls of the placenta, allowing blood to flow between the twins and increasing their risk of TTTS.

The risk of complication increases for monochorionic-monoamniotic twins because their umbilical cords may become entangled in the uterus.

Prenatal diagnosis of monochorionic twin anomalies
Monochorionic twin pregnancies are typically detected during a routine prenatal ultrasound. The best results are visible in ultrasounds conducted prior to 14 weeks of pregnancy, when the structures of the placenta and amniotic sac are most visible. The ultrasound will show a single placenta supplying blood flow to two fetuses 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.

Depending on the ultrasound findings, obstetricians will likely refer the mother to a perinatologist or maternal-fetal medicine specialist who specializes in high-risk pregnancies. Additional maternal and fetal tests may be required to screen for anomalies such as TTTS and TRAP sequence. These tests include fetal echocardiography, a specialized ultrasound of the heart and blood vessels that shows the flow of blood between the twins.

Testing and ultrasounds will likely be repeated throughout the pregnancy to monitor the size of the babies, blood supply and amniotic fluid.

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.

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. For monochorionic twins, the risk of low birth weight is four times higher than in pregnancies in which each fetus has its own placenta.

Weight inequality also can be a concern. A weight difference of 20 percent or more 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.

Unequal amniotic fluid levels can affect the twins. Twins deprived of a blood supply will produce too little amniotic fluid or oligohydramnios, limiting their movement and bladder size, among other consequences. A larger than normal blood supply will result in excess amniotic fluid or polyhydramnios, enlarged bladder and the possibility of hydrops.

Cord entanglement or compression is a risk for all monochorionic-monoamniotic twins, who share the same space in the amniotic sac. Umbilical cords are the lifeline to blood and nutrients for the 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.

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 will follow those for mothers carrying twins. Nutrition, rest, prenatal visits and other factors will be adjusted based on needs. Close observation by ultrasound examinations and other tests will be required throughout pregnancy to monitor the health of the twins. Bed rest, tocolytic medications and other measures may be recommended to prevent early delivery. Some mothers may need to be hospitalized for constant monitoring, as in the case of cord entanglement, TTTS and TRAP sequence.

Labor and delivery of monochorionic twins 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 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.

In the case of cord entanglement, physicians will recommend delivery of the twins as soon they are mature enough to survive, which may be as early as 24 weeks. In these cases, corticosteroid medications may be given to the mother to accelerate lung development in the twins so they have a greater chance of survival outside of the womb.

If excessive amniotic fluid is a concern, physicians may perform an amnioreduction to reduce the amount of amniotic fluid. This procedure will make the mother more comfortable, reduce the chance of premature labor and better equalize the flow of blood between the fetuses. It may need to be repeated throughout the 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 or laser ablation. The procedure is only offered at select treatment centers like the Fetal Concerns Center of Wisconsin. 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.

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

RFA is a procedure performed by our team of experts at 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. What about after 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. After these procedures, mothers will typically be admitted to the hospital for monitoring for two to three days. They are then discharged for ongoing care by their obstetrician. Bed rest may be recommended and medications prescribed to prevent premature delivery.

Babies born prematurely or with special needs will be cared for in a special care nursery called a neonatal intensive care unit (NICU), where expert physicians and staff can quickly respond if specialized care is needed.

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.

Many mothers are able to successfully breast feed their twins. Lactation specialists can help mothers of multiples learn techniques for breastfeeding their babies separately and together, and to 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.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 mothers more time to get to know their 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 for some length of time. How long your babies remain hospitalized will vary depending on their conditions. Once babies are able to feed, grow and stay warm they can usually be discharged.

What is my babies' longterm prognosis?
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 mental retardation, cerebral palsy, vision loss and hearing loss. 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.