Twin-twin transfusion syndrome during the 2nd trimester is associated with small intertwin hemoglobin differences.

In four twin pregnancies presenting with acute 2nd-trimester polyhydramnios due to presumed twin-twin transfusion syndrome, the fetal hemoglobin concentration was measured in samples obtained by cordocentesis. In no instance did the twin pair difference in the hemoglobin concentration exceed 2.7 g/dl.