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.


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). 


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

Radiographic features

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:


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

Ultrasound - obstetric

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