Monochorionic diamniotic twin pregnancy

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

Epidemiology

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.

Pathology

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

Ultrasound

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 2^nd 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:

• absent twin peak sign: differentiating from a DCDA pregnancy

Complications

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

• problems related to abnormal placental vascular anastomoses
  • twin to twin transfusion syndrome: can occur in ~15 ¹ - 30 ⁴ % of MCDA pregnancies
  • twin embolisation syndrome
  • twin reversed arterial perfusion sequence:
• demise of one twin: often associated with some adverse outcome to the other twin
• placental insertion related problems
  • increased incidence of velamentous cord insertion (c.f singleton pregnancy)
  • increased incidence of marginal cord insertion(c.f singleton pregnancy)

See also

• multifetal pregnancy
• monochorionic pregnancy