 The aim of this study is to compare the performance of this new noninvasive diagnostic test with the standard test, amniocentesis for amniotic fluid deltaOD450 values. In an international multicenter setting, a large cohort of consecutive Rh-alloimmunized pregnancies will be studied prospectively. Both diagnostic tests will be performed simultaneously in all patients, and compared with the gold standard, fetal blood sampling FBS to assess the actual hemoglobin concentration. 