monochorionic - diamniotic twins

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.

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 .

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 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 (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, post-zygotic non-disjunction can occur

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 - 30 % of MCDA pregnancies
  • 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 (c.f singleton pregnancy)
  • increased incidence of marginal cord insertion (c.f singleton pregnancy)

See also

• multifetal pregnancy
• monochorionic pregnancy

Siehe auch:

• Chorion
• marginal cord insertion
• Dottersack
• monochorionic twin pregnancy
• twin embolisation syndrome
• inter twin membrane
• dichorionic - diamniotic
• twin reversed arterial perfusion sequence
• monozygotic twins
• twin to twin transfusion syndrome (TTTS)
• Mehrlingsschwangerschaft
• Insertio velamentosa
• twin peak sign
• MCMA

und weiter:

• Zwillingsschwangerschaft
• monochoriale-monoamniote Zwillingsschwangerschaft