This site is targeted at medical and radiology professionals, contains user contributed content, and material that may be confusing to a lay audience. Use of this site implies acceptance of our Terms of Use.

Monochorionic diamniotic twin pregnancy

A monochorionic-diamniotic (MCDA) twin pregnancy is a sub type of monozygotic twin pregnancy. These pregnancies share a single chorion.


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.


A MCDA pregnancy results from a separation of a single zygote at ~4-8 days (blastocyst) 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 a 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 a 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

Related articles

Ultrasound - obstetric

Updating… Please wait.


Error Unable to process the form. Check for errors and try again.

Alert_accept Thank you for updating your details.