TAPS and TOPS: two distinct forms of feto-fetal transfusion in monochorionic twins.

Monochorionic twins share a single placenta with inter-twin vascular anastomoses, allowing the transfer of blood from one fetus to the other and vice versa. These anastomoses are the essential anatomical substrate for the development of severe complications, including twin-twin transfusion syndrome (TTTS) and twin-anemia-polycythemia sequence (TAPS). TTTS and TAPS are both chronic forms of feto-fetal transfusion. TTTS is characterized by the twin oligo-polyhydramnios sequence (TOPS), whereas TAPS is characterized by large inter-twin hemoglobin differences in the absence of amniotic fluid discordances. TAPS may occur spontaneously in a minority of monochorionic twins or in TTTS cases after laser treatment. This review focuses on the differences between TAPS and TTTS in terms of pathogenesis, incidence, diagnostic criteria, treatment modalities, perinatal outcome and long-term outcome.