From the Guidelines
No, heterozygous twins are not necessarily Dichorionic Diamniotic and homozygous twins are not necessarily Monochorionic Diamniotic. The correct terminology is that dizygotic (fraternal) twins are typically dichorionic diamniotic (DCDA), developing from two separate fertilized eggs with separate placentas and amniotic sacs. Monozygotic (identical) twins can have various placental and amniotic arrangements depending on when the fertilized egg splits. If splitting occurs within the first 3 days after fertilization, they will be dichorionic diamniotic (DCDA) 1. If splitting occurs between days 4-8, they will be monochorionic diamniotic (MCDA). If splitting occurs between days 8-13, they will be monochorionic monoamniotic (MCMA). The terms heterozygous and homozygous refer to genetic allele pairs rather than twin types.
These distinctions are important for understanding twin development and associated pregnancy risks, as monochorionic twins share a placenta and have higher risks of complications like twin-to-twin transfusion syndrome. According to the most recent evidence, monochorionic diamniotic pregnancies have an overall mortality rate of approximately 10%, primarily related to TTTS and discordant fetal anomalies 1.
Key points to consider:
- Dizygotic twins are typically dichorionic diamniotic (DCDA)
- Monozygotic twins can be dichorionic diamniotic (DCDA), monochorionic diamniotic (MCDA), or monochorionic monoamniotic (MCMA) depending on the timing of egg splitting
- Monochorionic twins have a higher risk of complications like twin-to-twin transfusion syndrome
- The terms heterozygous and homozygous refer to genetic allele pairs, not twin types. It is essential to determine chorionicity and amnionicity in twin pregnancies to assess the risk of complications and provide appropriate management, as recommended by the Society for Maternal-Fetal Medicine 1.
From the Research
Twin Pregnancy Types
- Heterozygous twins are typically Dichorionic Diamniotic (DCDA), meaning they have two separate placentas and two separate amniotic sacs 2.
- Homozygous twins, on the other hand, are usually Monochorionic Diamniotic (MCDA), meaning they share a single placenta but have two separate amniotic sacs 3, 4.
Characteristics of Monochorionic Diamniotic Twins
- Monochorionic diamniotic twins occur in about 1 in 300 pregnancies and face increased risks due to the shared circulation 3.
- They are at a higher risk of complications such as twin-twin transfusion syndrome and twin anemia polycythemia sequence 3, 4.
- Monochorionic twins are also more likely to have structural anomalies and growth restrictions compared to dichorionic twins 4, 5.
Comparison of Monochorionic and Dichorionic Twins
- Monochorionic twins have a higher rate of perinatal morbidity and mortality compared to dichorionic twins 5, 6.
- Dichorionic twins, on the other hand, have a lower risk of complications and better neonatal outcomes 2, 6.
- The type of twin growth, whether symmetric or discordant, also plays a significant role in determining the outcome of twin pregnancies 6.