Twin anemia–polycythemia sequence (TAPS) is a rare but serious complication unique to monochorionic twin pregnancies. It results from chronic, slow transfusion of blood through a limited number of very small placental vascular anastomoses, leading to progressive anemia in the donor twin and polycythemia in the recipient twin. Unlike twin-to-twin transfusion syndrome (TTTS), TAPS typically occurs in the absence of amniotic fluid discordance, making diagnosis more challenging.
The underlying pathophysiology is characterized by an unbalanced placental vascular architecture consisting predominantly of minute arteriovenous anastomoses with absent or insufficient compensatory arterioarterial connections. Antenatal diagnosis relies primarily on Doppler assessment of the middle cerebral artery peak systolic velocity (MCA-PSV), while postnatal confirmation is based on intertwin hemoglobin differences, reticulocyte count ratios, and placental examination.
Management strategies include expectant monitoring, intrauterine transfusion, partial exchange transfusion, and fetoscopic laser coagulation. Among these options, laser coagulation is the only treatment that addresses the underlying placental vascular pathology. The introduction of the Solomon laser technique has significantly reduced the incidence of residual anastomoses and post-laser TAPS.
Despite advances in diagnostic criteria and therapeutic approaches, TAPS remains underdiagnosed and poses significant challenges in clinical management. Further research is needed to optimize screening protocols, refine classification systems, and establish evidence-based treatment guidelines to improve perinatal outcomes in affected pregnancies.