An obstetric patient, 34 weeks pregnant, shows a positive antibody screen at the indirect antiglobulin phase of testing. She is group B, Rh negative. This is her first pregnancy. She has no prior history of transfusion. What is the most likely explanation for the positive antibody screen?
A. She has developed an antibody to fetal red cells.
B. She probably does not have antibodies because this is her first pregnancy, and she has not been transfused; check for technical error.
C. She received an antenatal dose of RhIg.
D. Impossible to determine without further testing.