monochorionic - monoamniotic twins

A monochorionic monoamniotic (MCMA) twin pregnancy is a subtype of monozygotic twin pregnancy.  These fetuses share a single chorionic sac, a single amniotic sac, and, in general, a single yolk sac.

Epidemiology

It accounts for the minority (~5%) of monozygotic twin pregnancies and ~1-2% of all twin pregnancies. The incidence is ~1 in 10000 of all pregnancies .

Pathology

It results from a separation of a single ovum at ~ 8-13 days following formation (i.e. later than with an MCDA pregnancy). By this time a trophoblast has already formed, yielding a single placenta. These fetuses share a single chorionic sac, a single amniotic sac, and most often a single yolk sac. The twins are identical (and of course of the same gender).

Associations

This type of pregnancy carries a relatively high incidence of congenital anomalies .

Radiographic features

Ultrasound

First trimester

• shows a twin pregnancy with a single gestational sac and a most often a single yolk sac (which helps to differentiate from a DCDA and MCDA pregnancy)
• there is no inter-twin membrane: theoretically, this differentiates from a DCDA and MCDA pregnancy
  • however, even in an MCDA pregnancy, the intertwin membrane may be difficult to see
  • therefore non-visualization of the intertwin membrane is not in itself diagnostic

Second trimester

Features noted on a second-trimester scan includes:

• specific to an MCMA pregnancy
  • there can be the presence of cord entanglement
  • there can be the presence of cord fusion
  • absent inter-twin membrane: although may be difficult to see sometimes even with an MCDA pregnancy
• common to both MCMA and MCDA pregnancies
  • a single placenta is seen 
  • absent twin peak sign 

Complications

An MCMA pregnancy carries the highest level of potential complications out of all twin pregnancies (with reported rates of overall perinatal mortality up to 70-80% ). These include:

• problems related to abnormal placental vascular anastomoses
  • twin to twin transfusion syndrome: his particular complication only occurs in ~10-15% of MCMA pregnancies and therefore less common than 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)
• umbilical cord related complications
  • umbilical cord knots
  • umbilical cord thrombosis

See also

• multifetal pregnancy
  • MCDA pregnancy
  • DCDA pregnancy

Siehe auch:

• marginal cord insertion
• Dottersack
• twin embolisation syndrome
• inter twin membrane
• monochoriale-diamniote Zwillingsschwangerschaft
• dichorionic - diamniotic
• twin reversed arterial perfusion sequence
• umbilical cord knots
• monozygotic twins
• entangled cords
• monochorionic - diamniotic twins
• twin to twin transfusion syndrome (TTTS)
• Mehrlingsschwangerschaft
• Insertio velamentosa
• twin peak sign
• Thrombose der Umbilikalarterien

und weiter:

• Zwillingsschwangerschaft
• monozygotic twin pregnancy