Birth Defects in Monochorionic Twin
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:
- Birth weight differences.
- Amniotic fluid problems.
- Umbilical cord entanglement or compression.
- Twin-to-twin transfusion syndrome (TTTS).
- Twin reversed arterial perfusion sequence (TRAP sequence).
- Increased risk of birth defects in each twin.
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.
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.
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.
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.
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.