Monochorionic twins

Detailed information on monochorionic twins

Monochorionic twins are a type of twin pregnancy where the twins share a single chorion, the outer membrane that surrounds the amniotic sac and the developing embryos. This condition occurs in monozygotic twins, which are twins that originate from a single fertilized egg that splits into two embryos. Monochorionic twins are always identical twins.

Development

Monochorionic twins develop when the fertilized egg splits between 4 to 8 days after fertilization. This timing results in the twins sharing a single placenta, which can lead to unique complications not seen in dichorionic twins, who have separate placentas.

Complications

Monochorionic twin pregnancies are at higher risk for certain complications due to the shared placenta. These include:

• Twin-to-twin transfusion syndrome (TTTS): A condition where blood flows unevenly between the twins through shared placental vessels, leading to one twin receiving too much blood and the other too little.
• Twin anemia-polycythemia sequence (TAPS): A form of TTTS where there is a slow, chronic transfusion of blood from one twin to the other, leading to anemia in one twin and polycythemia in the other.
• Selective intrauterine growth restriction (sIUGR): Occurs when one twin receives less blood flow and nutrients, resulting in restricted growth compared to the co-twin.
• Twin reversed arterial perfusion (TRAP) sequence: A rare condition where one twin, the "acardiac twin," lacks a functioning heart and is perfused by the other twin, the "pump twin."

Diagnosis

Monochorionic twins are typically diagnosed through ultrasound imaging. Key ultrasound findings include:

• A single placenta
• Absence of a thick dividing membrane between the twins
• "T-sign" at the junction of the amniotic membranes and placenta, indicating a monochorionic, diamniotic pregnancy

Monoamniotic twins at 15 weeks gestation

Management

Management of monochorionic twin pregnancies involves close monitoring to detect complications early. This includes frequent ultrasounds to assess growth, amniotic fluid levels, and Doppler studies of the umbilical arteries. In cases of TTTS, treatment options such as laser photocoagulation of placental vessels may be considered.

Prognosis

The prognosis for monochorionic twins depends on the presence and severity of complications. With early detection and appropriate management, outcomes can be improved. However, these pregnancies are generally considered high-risk and require specialized care.

Also see

• Twin-to-twin transfusion syndrome
• Monoamniotic twins
• Dichorionic twins
• Multiple birth