Numerous studies have shown that, if administered correctly, RhIg is effective at preventing D immunization. To work, RhIg must be given in sufficient dose, and it must be given before Rh immunization has begun.
Unfortunately, despite RhIg's proven efficacy, some women continue to make anti-D in the perinatal period. Such 'failures' are mainly (but not totally) due to human error.
Examples of how women may still produce anti-D some 40+ years after the implementation of RhIg prophylaxis:
- Immunization to D occurred before the administration of RhIg, e.g., before 28 weeks gestation*;
- Immunization to D occurred after the administration of RhIg at 28 weeks and before delivery because an antenatal fetomaternal hemorrhage (FMH) occurred that was too large for residual passive anti-D to give protection;
- Female was already immunized from a prior pregnancy but anti-D was too weak to be detected in antibody screen tests prior to RhIg administration;
- RhIg dosage was insufficient to clear a larger fetal bleed at delivery (e.g., FMH screen was not done or a false negative occurred);
- Incorrect calculation of RhIg dosage;
- RhIg administered too late , e.g., well after 72 hours of delivery;
- Antenatal RhIg not given, e.g., mother had no, or limited, access to prenatal care, or did not seek it, and a FMH occurred during pregnancy;
- Failure of physician to carry out prenatal blood testing;
- RhIg not given due to laboratory clerical or technical error in Rh typing the mother or child;
- RhIg not given in cases such as abortions, ectopic pregnancies, and trauma (e.g., car accidents).
* Because anti-D production before 28 weeks is rare (the order of 0.24% to 0.31%), RhIg's use earlier in pregnancy is not recommended. It is not cost effective and would expose most women to an unneeded blood product.