Monochorionic, diamniotic (MCDA) twins are the product of a single fertilized ovum (egg), resulting in genetically identical offspring. MCDA twins share a single placenta (blood supply) but have separate amniotic sacs.
The occurrence of MCDA twins occurs at a rate of 3 to 4 in 1,000 live births.
MCDA twin pregnancies are more frequently monitored throughout pregnancy in order to maximize the chances for a good outcome. In addition to complications such as preterm labor and preterm delivery that all twins face, MCDA twins are at risk for complications specific to a shared blood supply. Circulatory imbalance across blood vessels linking twins places them at risk for twin-twin transfusion syndrome (TTTS). Uneven division of the placenta may result in unequal placental sharing (UPS). Given an association between MCDA twins and physical malformations, MCDA twins undergo fetal echocardiography in addition to usual ultrasound-based assessments.
Though MCDA twins are at a higher risk for serious complications when compared to singleton pregnancies or fraternal (dichorionic) twin pregnancies, the majority are delivered without any significant complications. MCDA twins are usually born prematurely, even if no complications are identified before birth.