A Kleihauer test is used to confirm transplacental blood loss from fetus to mother. The test is performed on the mother's blood; the blood undergoes acid elution and staining.
Fetal red cells contain HbF and are therefore more resistant to acid elution than the maternal cells. The fetal cells therefore stain red, maternal cells stain pink.
Note that the Kleihauer test may be falsely negative if there is haemolysis of the fetal cells in the maternal circulation - ie blood group incompatibility.