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Monochorionic monoamniotic twin pregnancy

monochorionic-monoamniotic (MCMA) twin pregnancy is a sub type of monozygotic twin pregnancy. It accounts for the minority (~ 5%) of monozygotic twin pregnancies and ~ 1 - 2 % of all twin pregnancies. The incidence is ~ 1 in 10000 of all pregnancies 2.

Pathology

It results from a separation of a single ovum at ~ 8 - 13 days following formation (i.e. later than with an MCDA pregnancy). By this time a trophoblast has already formed, yielding a single placenta. These fetuses share a single chorionic sac, a single yolk sac and a single amniotic sac. The twins are identical (and of course of the same gender). 

Associations

This type of pregnancy carries a relatively high incidence of congenital anomalies 4.

Radiographic features

Ultrasound
First trimester
  • shows a twin pregnancy with a single gestational sac and a single yolk sac (differentiating from a DCDA and MCDA pregnancy).
  • there is no inter twin membrane : theoretically this differentiates from a DCDA and MCDA pregnancy. 
    • however, even in a MCDA pregnancy the intertwin membrane may be difficult to see.  
    • therefore non-visualisation of the intertwin membrane is not in itself diagnostic.
Second trimester

Features noted on a 2nd trimester scan includes

Complications

A MCMA pregnancy carries the highest level of potential complications out of all twin pregnancies (with reported rates of overall perinatal mortality up to 70 - 80 % 1). These include

See also

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