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Monochorionic diamniotic twin pregnancy

A monochorionic-diamniotic (MCDA) twin pregnancy is a sub type of monozygotic twin pregnancy. These pregnancies share a single chorion.

Epidemiology

It accounts for the vast majority (~ 70-75%) of monozygotic twin pregnancies although only  ~ 30 % of all twin pregnancies. The estimated incidence is at ~ 1:400 pregnancies 11.

Pathology

A MCDA pregnancy results from a separation of a single zygote at ~ 4-8 days (blastocyst) following formation. These fetuses share a single chorionic sac but two yolk sacs and two amniotic sacs. The twins are identical (and of course of the same gender). By this time a trophoblast has already formed yielding a single placenta.

Radiographic features

Ultrasound
First trimester
  • shows a twin pregnancy with a single gestational sac, and almost always two separate yolk sacs 9-10 (differentiating from a MCMA pregnancy).
  • a thin inter twin membrane may be seen.
Second trimester

Findings noted on a 2nd trimester scan includes

  • often a single placenta is seen : differentiating from a DCDA pregnancy
  • inter twin membrane 
    • present : differentiating from a MCMA pregnancy
    • but appears very thin : differentiating from a DCDA pregnancy (although this assessment becomes increasingly difficult with the progression of pregnancy)

Negative findings

Complications

Potential complications that can occur with this type of pregnancy includes

See also

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