Rh Alloimmunization Prophylaxis
A 28-year-old woman at 20 weeks gestation reports painless vaginal spotting for one day with no trauma or intercourse. Her blood group is O negative and the antibody screen is negative. Ultrasound shows an appropriately grown fetus, a fundal placenta, and a 38 mm closed cervix. Which of the following is the most appropriate next step?
- A.Discharge without further treatment
- B.Administer anti-D immunoglobulinCorrect
- C.Admit for 24 hours of observation
- D.Continue prenatal vitamins only
Explanation
Any Rh D-negative, non-sensitised woman who bleeds vaginally during pregnancy should receive anti-D immunoglobulin, and the reassuring ultrasound does not change this. Even small fetomaternal haemorrhages can expose the mother to fetal D antigen and trigger production of anti-D antibodies, which cross the placenta in this and future pregnancies and cause haemolytic disease of the fetus and newborn. Because sensitisation cannot be reversed once it occurs, prophylaxis is deliberately generous. Anti-D works by clearing D-positive fetal red cells before the maternal immune system can mount a primary response, so it should be given within 72 hours of the bleeding event. Beyond antepartum bleeding, the standard indications are abdominal trauma, external cephalic version, invasive procedures such as amniocentesis and chorionic villus sampling, ectopic pregnancy, miscarriage or termination, and delivery of a D-positive infant. Routine antenatal prophylaxis is also given at 28 weeks, and after 20 weeks a Kleihauer-Betke or flow cytometry test quantifies the fetomaternal bleed so that additional doses can be given if it exceeds the standard 300 microgram dose coverage. Discharging her, simply observing, or continuing vitamins all miss a preventable and permanent complication. A negative antibody screen confirms she is not already sensitised and remains a candidate for prophylaxis.
Why each option
- A.
- Discharging her without anti-D leaves a preventable risk of alloimmunisation that cannot be undone once antibodies form.
- B.
- Correct. Antepartum bleeding in an Rh-negative, non-sensitised woman is an indication for anti-D immunoglobulin within 72 hours.
- C.
- Observation alone addresses neither the sensitisation risk nor any diagnostic question, since the ultrasound is already reassuring.
- D.
- Vitamins are irrelevant to the immunological risk created by the bleeding episode.
Reference: ACOG Practice Bulletin 181 (reaffirmed 2023), Prevention of Rh D Alloimmunization
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