Fetal hemoglobin changes in the twin oligohydramnios-polyhydramnios sequence.

Fetal hemoglobin concentration was measured in each twin from five monochorionic pregnancies complicated by twin-to-twin transfusion, diagnosed by growth discordance and coexistent oligohydramnios-polyhydramnios. In three cases the hemoglobin concentration of the smaller twin with oligohydramnios was lower than the normal mean for gestation, and the hemoglobin difference was more than 5 g/dl in two of these cases. In the other two cases, the hemoglobin concentration of the four fetuses with coexistent oligohydramnios-polyhydramnios was within the normal range. This suggests that twin-to-twin transfusion may occur without hemoglobin changes, or that alternatively coexistent oligohydramnios-polyhydramnios occurs in the absence of twin-to-twin transfusion.