How probable is it that a pregnant woman would receive a false positive hiv screening?

Still very unlikely: On average, 0.2% of elisa tests give a false positive result that are later proven incorrect by a confirmatory western blot test. Causes of false positive HIV antibody tests include autoimmune disorders, recent vaccinations, multiple pregnancies. This is because these conditions can stimulate the body's production of antibodies that can cross react with HIV antigens causing false positive result.