 It has been modified to distinguish fetal from maternal hemoglobin in blood samples from any source.The Apt test is most commonly used in cases of vaginal bleeding late during pregnancy (antepartum haemorrhage) to determine if the bleeding is from the mother or the fetus.In practice, the Apt test may not be done when there is suspicion of vasa previa, because the time to fetal collapse with bleeding from vasa previa is often very short.