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. The twins are identical (and of course of the same gender). By this time a trophoblast has already formed yielding a single placenta.
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 2nd trimester scan includes
- often a single placenta is seen : differentiating from a DCDA pregnancy
- inter twin membrane
Negative findings
- absent twin peak sign - differentiating from a DCDA pregnancy
Complications
Potential complications that can occur with this type of pregnancy includes
- 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)
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