This test assist in determining whether a woman has Rh-positive or Rh-negative blood (Rh antibody titer) is done early in pregnancy. The Rhesus factor is important during pregnancy and causes problems when an Rh-negative persons blood comes in contact with Rh-positive blood.If you have Rh-negative blood and have been exposed to Rh-positive blood, your immune system may make antibodies to attack the Rh-positive blood. When a pregnant woman has these antibodies, it is called Rh sensitization.