The prevalence and timing of seroconversion during pregnancy-A retrospective study to enable safe transfusion.

Background: During pregnancy, exposure to fetal red blood cell antigens can trigger alloimmunization. To enable safe transfusion of compatible blood for pregnant individuals where urgent surgery might be necessary, blood bank standards mandate a valid blood type and antibody screening every 72 h ("3-day rule"). This practice requires frequent blood drawings contributing to patient discomfort and costs. To investigate if this practice is required, we evaluated the proportion of pregnant individuals who develop clinically significant alloantibodies within 72 h of a previous negative antibody screen.

Study design and methods: Retrospective cohort of RhD-positive pregnant individuals with initial negative antibody screens, undergoing repeated screens at variable intervals. Seroconversion rates were compared with healthy male blood donors. Characteristics of pregnant individuals with and without seroconversion were analyzed.

Results: Among 8659 RhD-positive pregnant individuals with initial negative antibody screens, 56 (0.6%) converted to a positive antibody screen, while only 15 (0.2%) developed clinically significant alloantibodies. Of these 15, only one converted within 3 days of testing, while others converted within 5-95 days. Two of the 15 women received a blood transfusion, 8-97 days before seroconversion. No male donors seroconverted during 9 months of follow up. Pregnant individuals with seroconversion were more likely to have systemic lupus erythematosus and antiphospholipid antibody syndrome.

Discussion: Only one pregnant individual developed clinically significant alloantibodies within 72 h of a negative screening, compared to none of the control group. For pregnant individuals without recent transfusion, extending antibody screening intervals to 1 week appears safe.