Placental vascular anastomoses in monochorionic twins may lead to acute or chronic inter-
twin transfusion. We report an uncommon form of chronic inter-
twin transfusion, referred to as twin
anemia-
polycythemia sequence (TAPS), with severe
anemia in one twin and
polycythemia in the other, without the characteristically associated twin oligo-
polyhydramnios sequence (
TOPS) seen in the classical
twin-to-twin transfusion syndrome (TTTS). The
clinical course and placental characteristics of two pairs of monochorionic twins with TAPS born at, respectively, 33 and 34 weeks' gestation were reviewed. Serial fetal ultrasound examinations revealed no signs of
TOPS. At birth, both donor twins were severely anemic requiring
blood transfusion and both recipients were polycythemic, one requiring partial volume exchange transfusions. Inter-twin difference in reticulocyte counts was extremely high, suggesting a chronic form of inter-twin
blood transfusion. Placental injection studies revealed a preponderance of very small (<1mm) arterio-venous anastomoses in one direction. In conclusion, chronic inter-
twin transfusion may lead to an uncommon form of inter-
twin transfusion, named TAPS, not associated with
TOPS, resulting in severe fetal or neonatal hematological complications. We hypothesize that TAPS is mediated through minuscule unidirectional anastomoses. TAPS can be diagnosed antenatally with Doppler studies and postnatally by
hemoglobin and reticulocyte measurements.