Monochorionic diamniotic twin pregnancy

Last revised by Joshua Yap on 2 Aug 2022

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) 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 layperson's term is that the twins are "identical". In reality, they are phenotypically similar, and of course of the same gender. 

  • shows a twin pregnancy with a single gestational sac, and almost always two separate yolk sacs 9,10 (differentiating from an MCMA pregnancy)
  • at 14-18 weeks, often a single placenta is seen: differentiating from a DCDA pregnancy
  • a thin inter-twin membrane may be seen
    • due to amnions abutting the placenta
    • present: differentiating from an MCMA pregnancy
    • but appears very thin without intervening chorion (often taken as <2 mm): differentiating from a DCDA pregnancy (although this assessment becomes increasingly difficult with the progression of pregnancy)
    • T-sign of the intertwin membrane

Findings noted on a second-trimester scan include:

  • the number of placental masses, thickness of the membrane, and the presence/absence of the twin-peak sign are still viable options for determining chorionicity
  • fetal sex
    • almost always the same sex
    • in rare circumstances, postzygotic non-disjunction can occur

Negative findings:

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

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Cases and figures

  • Case 1
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  • Case 2
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