Monochorionic monoamniotic twin pregnancy

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 ².

Associations

This type of pregnancy carries a relatively high incidence of congenital anomalies ⁴.

Pathology

It results from a separation of a single blastocyst at ~8-13 days following formation (i.e. later than with a monochorionic diamniotic (MCDA) twin 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). 

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 dichorionic diamniotic (DCDA) and MCDA twin 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 

Treatment and prognosis

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% ¹). 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 ⁷

  • 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)

• umbilical cord related complications

  • umbilical cord knots

  • umbilical cord thrombosis

See also

• multifetal pregnancy

  • MCDA pregnancy

  • DCDA pregnancy