Arterio-arterial anastomoses do not prevent the development of twin anemia-polycythemia sequence.

Monochorionic twin pregnancies are associated with increased perinatal morbidity and mortality. The vascular placental anastomoses in these pregnancies can cause severe complications. We describe a case of twin anemia-polycythemia sequence (TAPS) with an atypical placental angioarchitecture. During pregnancy serial ultrasound examinations of both twins revealed no amniotic fluid discordance and no abnormal Doppler ultrasound measurements (umbilical cord pulsatility index and middle cerebral artery peak systolic velocity). The twins, born at 33 + 3 weeks gestation after spontaneous onset of labour, were found to be anemic and polycythemic, respectively. Placental examination with colored dye injection showed, apart from small ateriovenous anastomoses, an arterio-arterial anastomosis. As arterio-arterial anastomoses have not been described in cases with spontaneous TAPS to date, it was postulated that such anastomoses carried a protective effect against the development of this complication.