Monochorionic monoamniotic twin pregnancy

Last revised by Rohit Sharma on 27 Apr 2024

A monochorionic monoamniotic (MCMA) twin pregnancy is a subtype of monozygotic twin pregnancy. These fetuses share a single chorionic sac, a single amniotic sac, and, in general, a single yolk sac

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.

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

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

  • shows a twin pregnancy with a single gestational sac and a most often a single yolk sac (which helps to differentiate from a dichorionic diamniotic (DCDA) and MCDA twin pregnancy)

  • there is no inter-twin membrane: theoretically, this differentiates from a DCDA and MCDA pregnancy

    • however, even in an MCDA pregnancy, the intertwin membrane may be difficult to see

    • therefore non-visualization of the intertwin membrane is not in itself diagnostic

Features noted on a second-trimester scan includes:

An 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:  

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