Twin pregnancies are interesting yet complex concepts in human development, often sparking great curiosity. A crucial part of twin pregnancies is understanding how they differ based on placentation and amniotic sac models.
Dichorionic, Diamniotic, and Monochorionic twin pregnancies, as per the Janoschek classification, differ significantly in structure, management approach, and potential risks. This review discusses their differences with an eye toward their implications for prenatal evaluation and results.
Twins are identified by chorion (outer membrane) and amnion (inner membrane). There are mainly two categories:
In DCDA pregnancies, the placenta and amniotic sac develop separately. This condition is found in all fraternal (dizygotic) twins and some identical (monozygotic) twins if the early cleavage occurs within the first three days after fertilisation. While this separation is normal, it can contribute to pregnancy-related complications.
Monochorionic twins share a placenta and are always identical (monozygotic), resulting from a single fertilised egg splitting between days 4 and 8 of embryonic development. These pregnancies are further classified based on whether the twins share the same amniotic sac (monoamniotic) or have separate sacs (diamniotic).
MCDA twins are identical twins that arise from a single fertilised egg that divides early in development.
A high-risk condition known as MCMA (monoamniotic) involves only a single placenta and single amniotic sac. These differences significantly influence prenatal monitoring and management strategies.
Take a look at the differences in Monochorionic vs dichorionic twin pregnancies to have a better understanding of the topic:
● DCDA Twins: DCDA twins have two separate placentas, ensuring independent blood and nutrient supplies. This eliminates the risks associated with sharing blood or nutrients.
● Monochorionic Twins: Placenta sharing can cause morbidity. For example, Twin-to-Twin Transfusion Syndrome (TTTS) is a condition in which asymmetric circulation is established in one twin, typically at the expense of the other. This disease requires careful surveillance and, at times, expert medical treatments.
● Diagnosis DCDA Twin Pregnancy Ultrasound: Initial ultrasounds include two separate gestational sacs, each with its pair of chorionic membranes (CM). The "lambda sign," known as the "twin peak symbol," determines DCDA pregnancies, especially early pregnancy.
● Monochorionic Twin Ultrasound: The single-layered chorionic membrane is identified, and a "T-sign" is observed at the site of the fusion of the amniotic membranes. Serial ultrasounds enable us to image fetal growth, vascularisation, and options within fetal growth and developmental abnormalities.
These pregnancies usually have fewer complications. However, they can still have risks like preterm birth, growth differences, and gestational hypertension, though these are less common.
TTTS: Twin-to-Twin Transfusion Syndrome (TTTS) is a complication observed in about 10–15% of monochorionic pregnancies and has to be diagnosed early in pregnancy with a USG.
● Selective Intrauterine Growth Restriction (sIUGR): Asymmetric placental distribution can limit the growth of one twin, increasing the situation's complexity.
● Cord Entanglement: In the MCMA monozygotic twin (twins) pair, the risk for umbilical cord complications is more significant due to a shared amniotic membrane.
● DCDA Twins: Growth patterns vary depending on the placental support available to each twin.
● Monochorionic Twins: Uneven placental distribution leads to size differences, requiring closer monitoring. Risk of Congenital birth defects is higher in Monochorionic twins.
● Risk of medical disorders like Anemia, Gestational Hypertension ,Gestational Diabetes etc, Complicating the twin Gestational is higher than the singleton pregnancy
How can one ensure that twin pregnancies are carried to full term safely? Effective clinical management can help.
● DCDA Twins: Like singleton pregnancies, monitoring is scheduled regularly between 1 and 30 weeks of gestation during early pregnancy. The frequency typically increases during the second and third trimesters to ensure normal development and detect potential issues.
● Monochorionic Twins: These pregnancies require specialised monitoring, including reviewing ultrasounds at 2–3 week intervals beginning at 16 weeks to closely monitor fetal growth, identify TTTS, or evaluate amniotic fluid.
● DCDA Twins: Delivery is usually planned for 37–38 weeks if there are no complications. Vaginal delivery is possible in many cases, depending on fetal presentation.
● Monochorionic Twins: Delivery is often scheduled in advance, usually between 34–36 weeks of gestation, to minimise risks associated with placental sharing. Caesarean delivery is strongly recommended for MCMA twin pregnancies to prevent complications during labour.
For monochorionic twins, fetoscopic laser surgery can treat TTTS by disconnecting abnormal blood vessels between the twins' shared placenta. In some cases, other procedures like transuterine transfusion or targeted reduction may not be considered effective treatments.
Advanced imaging techniques, like ultrasound for DCDA twin pregnancies, help doctors monitor and improve pregnancy outcomes. In monochorionic pregnancies, tools such as Doppler ultrasound allow doctors to check blood flow and predict potential complications, enabling early intervention and better care.
Understanding the type of twin pregnancy helps parents prepare better. Clear communication with doctors and personalised care plans are crucial to setting expectations and ensuring the babies' health. Whether it's a low-risk DCDA twin pregnancy or a high-risk monochorionic one, integrated care is vital for good outcomes.
Both DCDA (dichorionic diamniotic) and MC (monochorionic) twin pregnancies have their challenges and benefits, and understanding these differences is essential for proper care. DCDA twins usually have fewer complications because each twin has its own placenta and amniotic sac. MC pregnancies need more careful monitoring to avoid risks like TTTS, growth issues, and placental problems. Doctors aim to ensure the best possible outcomes for babies and their parents with proper monitoring and timely treatment.
Remember that every twin pregnancy is unique, and the care plan should be thoughtful and carefully prepared.
Monochorionic monoamniotic (MCMA) twin pregnancies are especially at risk for preterm delivery due to complications like twin-twin transfusion syndrome. However, all twin pregnancies are more likely to have early delivery than singleton pregnancies.
With proper medical care and monitoring, it's possible to manage monochorionic twin pregnancies to term. However, these pregnancies carry higher risks, including TTTS, selective intrauterine growth restriction (sIUGR), and cord abnormalities. Regular ultrasounds and early interventions can significantly improve outcomes.
Monochorionic twins may face chronic health issues, including neurological disorders like cerebral palsy, often due to complications such as TTTS or preterm delivery. Developmental abnormalities can also be a concern. However, with careful monitoring and control of intrauterine conditions, most monochorionic twins lead everyday lives with no significant long-term effects reported.
Yes, Dichorionic Diamniotic (DCDA) twins can be monozygotic if they result from splitting a single fertilised egg within the first 3 days of gestation. This early separation allows each twin to develop its own placenta and amniotic sac. However, DCDA twins are typically dizygotic.
