From prophylaxis to parturition: evaluating antibody screen reactivity following antenatal anti-D at 28 weeks.

OBJECTIVE

To assess the serological detectability of prophylactic anti-D immunoglobulin at term and identify factors associated with antibody loss in RhD-negative women who received a single 300-µg dose at 28 weeks' gestation.

METHODS

This retrospective cohort included 915 singleton RhD-negative pregnancies delivered at Kayseri City Hospital between January 2023 and January 2025. All women had a negative antibody screen at 24 weeks, received routine antenatal Rhesus immune globulin (RhIg) at 28 ± 2 weeks, and underwent indirect antiglobulin test (IAT) testing within 72 h before delivery. Group comparisons used independent-samples t tests and χ2 tests (normality by Kolmogorov-Smirnov); p < .05 was considered significant.

RESULTS

The mean maternal age was 28 ± 6 years and mean BMI 30.8 ± 4.4 kg m-2. Delivery occurred at 38 ± 2 weeks, 69 ± 15 days after prophylaxis. Anti-D was detectable in 286 women (33%), while 629 (67%) tested IAT-negative. IAT-positive women had received prophylaxis one week later (29 ± 2 vs 28 ± 1 weeks, p < .001) and delivered nine days sooner after injection (63 ± 16 vs 72 ± 13 days, p < .001). Maternal age, BMI, and neonatal RhD status were not associated with antibody persistence.

CONCLUSION

A single 300-µg anti-D dose at 28 weeks leaves nearly two-thirds of RhD-negative women serologically undetectable for anti-D at term once the prophylaxis-to-delivery interval exceeds 10 weeks. While this suggests a potential late-gestation coverage gap, it does not directly establish the level of immunologic protection or the risk of new alloimmunization. Therefore, strategies such as a second antenatal dose at 34-36 weeks, late-pregnancy IAT-triggered boosters, or BMI-adjusted dosing should be prospectively evaluated within the framework of existing national guidelines to further minimize residual RhD alloimmunization.