Having twins can be an exciting experience for expecting parents. However, carrying more than one baby also comes with increased risks compared to a singleton pregnancy. Some types of twin pregnancies have higher risks than others. Understanding the different types of twins and their potential complications is important for managing a twin pregnancy safely.
Types of Twins
There are two main types of twins – identical (monozygotic) and fraternal (dizygotic). Identical twins form when one fertilized egg splits into two embryos early in development. They share nearly 100% of their DNA and will always be the same sex. Fraternal twins form when two separate eggs are fertilized by two different sperm. They share around 50% of their DNA and can be the same or different sex.
Twins can also be classified based on their placentas and amniotic sacs:
- Dichorionic diamniotic (di/di) – Each twin has their own placenta and amniotic sac.
- Monochorionic diamniotic (mo/di) – Twins share one placenta but have separate amniotic sacs.
- Monochorionic monoamniotic (mo/mo) – Twins share one placenta and one amniotic sac.
Dichorionic twins can be identical or fraternal. Monochorionic twins are always identical. The type of placentation greatly impacts the risks associated with twin pregnancies.
Risk Factors
Some factors that increase the chance of having twins include:
- Family history – Having twins runs in families.
- Older maternal age – Women over 30 are more likely to release multiple eggs.
- Fertility treatments – Medications and procedures like IVF can increase the chance of multiples.
- Race – African American women have higher twin rates.
- Prior pregnancy – Having twins before makes subsequent twins more likely.
However, even in the absence of these factors, a spontaneous twin pregnancy can still occur. Let’s look at the risks associated with different twin types.
Dichorionic Diamniotic (Di/Di)
Dichorionic diamniotic (di/di) is the most common twin pregnancy, accounting for about 70% of twins. Each baby has their own placenta and amniotic sac. This type has lower risks compared to monochorionic twins but still has elevated risks compared to singleton pregnancies.
Risks
- Preterm birth – Around 60% of di/di twins are born prematurely before 37 weeks.
- Low birth weight – Average di/di twins weigh around 5 pounds each at birth.
- Preeclampsia – Risk is 2-3 times higher than with a singleton.
- Placental abruption – The placenta detaching too early affects up to 6% of di/di pregnancies.
- Stillbirth – Risk is 2-3 times higher than for a singleton pregnancy.
- Fetal growth restriction – One or both babies grow poorly, occurring in up to 15% of di/di twins.
- Congenital anomalies – Birth defects occur more often compared to singletons.
- Twin-twin transfusion syndrome – Unbalanced blood flow between twins through placental vessels occurs in 10-15% of di/di pregnancies.
Careful monitoring throughout pregnancy is important to catch problems early. Most di/di twins do well with proper prenatal care and management of complications if they arise.
Monochorionic Diamniotic (Mo/Di)
Monochorionic diamniotic (mo/di) twins account for around 20% of twin pregnancies. They share one placenta but each have their own amniotic sac. Mo/di twins have higher risks than di/di twins since complications affecting the shared placenta can impact both babies.
Risks
- Growth discordance – One twin receives less blood flow leading to impaired growth, affecting 10-15% of mo/di twins.
- Selective fetal growth restriction – Only one twin has growth problems, occurring in up to 25% of mo/di pregnancies.
- Twin-twin transfusion syndrome (TTTS) – Unbalanced blood flow through the shared placenta, affecting 10-15% of mo/di pregnancies.
- Placental insufficiency – The shared placenta cannot support both twins adequately.
- Preterm birth – Over 50% of mo/di twins deliver preterm.
- Congenital anomalies – Birth defects are more common compared to di/di twins.
- Stillbirth – Risk is higher than di/di pregnancies.
- Unequal placental sharing – One twin receives more of the placental mass, putting the other at risk.
Mo/di twins need to be followed closely for TTTS, growth problems, and other placental complications. Interventions may be needed in severe cases.
Monochorionic Monoamniotic (Mo/Mo)
Monochorionic monoamniotic (mo/mo) twins represent only 1-2% of twin pregnancies. They share one placenta and one amniotic sac. This is the highest risk twin type due to risks from umbilical cord entanglement and compressed blood flow.
Risks
- Cord entanglement – Cords can wrap tightly around each other, compressing blood flow to one or both twins.
- Cord compression – The cramped amniotic sac increases risk of a cord getting compressed.
- Preterm birth – Most mo/mo pregnancies deliver before 32 weeks.
- Fetal demise – Death of one or both twins occurs in 25-30% of mo/mo pregnancies.
- TTTS – Unbalanced blood flow through the placenta happens less often than in mo/di twins.
- Growth restriction – Due to compressed cord flow, up to 25% of mo/mo twins experience impaired growth.
Mo/mo twins have mortality rates up to 50% higher compared to other twin types. Close monitoring in a specialty center starting early in pregnancy is essential to manage this high-risk pregnancy.
Comparing the Risks
To summarize the risks:
| Twin Type | Key Risks |
|---|---|
| Dichorionic diamniotic (di/di) | Preterm birth, low birth weight, twin-twin transfusion syndrome, placental abruption |
| Monochorionic diamniotic (mo/di) | TTTS, growth problems, placental insufficiency |
| Monochorionic monoamniotic (mo/mo) | Cord accidents, preterm birth, fetal demise |
Mo/mo twins face the highest risks mainly due to cord entanglement and compression. Mo/di twins also face significant risks from an inadequate shared placenta. Di/di twins have the lowest risks but still greater than a singleton pregnancy.
Preventing Complications
Although the type of twin pregnancy cannot be controlled, certain steps can be taken to minimize risks:
- Seek care from a maternal-fetal medicine specialist, ideally starting in the first trimester.
- Undergo frequent ultrasounds and fetal testing to monitor for problems.
- Be evaluated for symptoms of preterm labor and get appropriate interventions if needed.
- Have cervical length assessed starting in the second trimester.
- Get screening for TTTS and growth discordance.
- Deliver at a specialty hospital equipped to handle higher-risk deliveries.
Following provider recommendations for extra monitoring and care is crucial for identifying complications early and managing them promptly. This gives the best chance for a healthy outcome.
Conclusion
Monochorionic monoamniotic (mo/mo) twin pregnancies carry the highest risks, mainly due to risks of umbilical cord accidents in the shared sac. Monochorionic diamniotic (mo/di) twins also face significant risks from having a shared placenta. Dichorionic diamniotic (di/di) twins have the lowest risks of the three twin types but still greater risks than singleton pregnancies. All twin pregnancies benefit from specialized maternal care to monitor for complications and intervene early when needed. Understanding the risks specific to each twin type allows parents and providers to be prepared and take appropriate precautions.