Monochorionic diamniotic twin pregnancy

A monochorionic diamniotic (MCDA) twin pregnancy is a subtype of monozygotic twin pregnancy. These fetuses share a single chorionic sac but have two amniotic sacs and two yolk sacs


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.


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

The layman term is that the twins are "identical" - in reality, they are phenotypically similar, and of course of the same gender. 

Radiographic features

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

Findings noted on a 2nd trimester scan include:

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

Negative findings:


Potential complications that can occur with this type of pregnancy include:

See also

Ultrasound - obstetric
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Article Information

rID: 13806
System: Obstetrics
Tag: cases
Synonyms or Alternate Spellings:
  • Monochorionic diamniotic twins
  • MCDA pregnancy
  • MCDA twin
  • Monochorionic-diamniotic (MCDA) twin pregnancy
  • Monochorionic-diamniotic (MCDA) pregnancy
  • Mono-chorionic di-amniotic twins
  • Mono-chorionic di-amniotic twin pregnancy
  • MDT pregnancy
  • MCDA twins
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