(984) Rate of clinically significant red blood cell antibody seroconversion in pregnancy

To determine the rate of clinically significant red blood cell (RBC) antibody seroconversion in pregnancy.

Study Design: This is a retrospective cohort study of all pregnant patients who delivered at a large multi-hospital system from 2016-2023. In this system, a type and screen is obtained at entry to prenatal care, at time of admission for delivery, prior to administration of Rh(D) immune globulin (RhIG), or as clinically indicated. Patients with missing RBC antibody screen at delivery or no record of type and screen at entry to prenatal care were excluded. All other patients with a positive screen at delivery were categorized as passively immunized due to RhIG, alloimmunized (antibody previously identified during the pregnancy), or seroconverted (antibody not previously identified in the pregnancy). Antibodies were described as clinically significant if associated with hemolytic disease of the fetus and newborn (HDFN) to any degree per ACOG Practice Bulletin 192.

Results:

There were 58,912 pregnant individuals with 71,384 eligible deliveries in the time period. 66,615 (93.3%) deliveries had negative RBC antibody screen at delivery and 4,176 (5.9%) deliveries had a positive antibody screen only due to passive immunity from RhIG. Of the remaining 593 (0.8%) deliveries, 351 (0.5%) had clinically significant antibodies associated with HDFN, and among those, 168 (0.2%) seroconverted in pregnancy. 148 (88.1%) of these seroconverted deliveries were Rh positive and 20 (11.9%) of them were Rh negative. The most common antibodies newly acquired in pregnancy were anti-E, anti-c, anti-JkA, anti-C, anti-C, anti-M, and anti-Kell (Figure). 

Conclusion:

Development of new clinically significant RBC antibody in pregnancy occurred at a rate of 0.2% in this large cohort study. Routine third trimester assessment of maternal antibody status may not be indicated due to low likelihood for new clinically significant seroconversion.