Twin to twin transfusion syndrome (TTTS): results from abnormalities of the placenta in monozygous twin pregnancies. TTTS results from blood passing disproportionately (transfusing) from one twin baby to the other through connecting blood vessels within the shared (monochorionic) placenta.
Monozygotic (monozygous) twins: twins formed from a single zygote.
Monochorionic twins: twins who have developed in a single chorionic sac. These twins are always monozygotic.
Yes, a very serious situation.