**9.5. Twin Anemia Polycythemia Sequence (TAPS)**

Twin anemia polycythemia sequence (TAPS) occurs spontaneously in up to 5% of all mono‐ chorionic pregnancies or even after fetoscopic laser photocoagulation, with an estimated prevalence of 13%. This syndrome is characterized by a substantial difference in hemoglobin levels among twins, in absence of discordance in the amniotic fluid. It could be mainly explained by the presence of few persistent arteriovenous anastomoses besides the reduced placental territory where the circulating blood is transferred from donor to the receiver twin, in a unidirectional flow [87].

Prenatal diagnosis may be assessed through the determination of the peak systolic velocity in the middle cerebral artery (PSV-MCA) by dopplervelocimetry. The anemic twin will have a PSV-MCA >1, 5MoM in contrast to a decreased PSV-MCA <0, 8MoM in the polycythemic cotwin [88]. In the postnatal period, diagnostic criteria are based on different levels of hemoglobin between fetuses over 8g/dL, reticulocytes amount over 1.7% or small anastomoses <1mm [89].

Treatment includes expectant management, labor induction, intrauterine blood transfusion (intravenous or intraperitoneal), selective feticide and fetoscopic laser coagulation. Survival rates up to 80% are achieved when identified in early stages, although there are no studies of long-term neurological outcome [87].
