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.
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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
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.
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 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
Second and third trimesters
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:
- absent twin peak sign: differentiating from a DCDA pregnancy
Treatment and prognosis
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 ~15-30% of MCDA pregnancies 1,4
- twin embolization 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 (cf. singleton pregnancy)
- increased incidence of marginal cord insertion (cf. singleton pregnancy)
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