Monochorionic monoamniotic twin pregnancy

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

Epidemiology

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 an 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:

  • specific to an MCMA pregnancy:
  • common to both MDMA and MDCA pregnancies

Complications

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

See also


Ultrasound - obstetric
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Article Information

rID: 13816
Tag: cases
Synonyms or Alternate Spellings:
  • Monochorionic monoamniotic twins
  • Monochorionic monoamniotic (MCMA) twins
  • MCMA twin pregnancy
  • MCMA twins
  • Monochorionic monoamniotic twins (MMT)
  • Monochorionic - monoamniotic twin pregnancy
  • Monochorionic - monoamniotic twins
  • Mono - Mono twins

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