Final answer:
Twin twin transfusion syndrome occurs in monozygotic (identical) twins sharing a single placenta, also known as monochorionic twins. It results from unequal blood passage between the twins due to abnormal vascular connections on the shared placenta surface.
Step-by-step explanation:
Twin twin transfusion syndrome (TTTS) is a condition that is present in monochorionic twin pregnancies, which involves twins that share a single placenta. This occurs specifically with monozygotic (identical) twins. TTTS arises due to abnormal vascular connections on the placenta's surface, allowing blood to pass unevenly between the twins. One twin, referred to as the donor twin, might provide the other, known as the recipient twin, with too much blood, leading to potential complications for both.These twins are much less commonly produced when a zygote divides into two separate offspring during early development, most commonly during the early blastocyst stage. As a result, the separated twin embryos will share a placenta and a chorionic cavity, which sets the stage for TTTS. Since the condition is associated with identical twins sharing a placenta, this context drastically differs from non-identical or fraternal twins, known as dizygotic twins, who have two separate placentas.