A Rh-negative woman at 28 weeks gestation with no prior sensitisation (indirect Coombs test negative) receives antenatal anti-D prophylaxis. The rationale for this prophylaxis is:
- A The anti-D crosses the placenta and eliminates Rh-positive fetal red cells from fetal circulation
- B Antenatal anti-D prevents development of anti-D antibodies in subsequent pregnancies only if given after 34 weeks
- C Silent fetomaternal haemorrhage of fetal Rh-positive cells can occur throughout pregnancy and antenatal anti-D prevents sensitisation from such occult bleeds ✓
- D Anti-D is given only after sensitising events and is not recommended routinely at 28 weeks
Correct answer: C. Silent fetomaternal haemorrhage of fetal Rh-positive cells can occur throughout pregnancy and antenatal anti-D prevents sensitisation from such occult bleeds
Explanation
Small amounts of fetal red cells cross into maternal circulation throughout pregnancy (occult fetomaternal haemorrhage), which can sensitise an Rh-negative mother against a Rh-positive fetus. Antenatal anti-D prophylaxis at 28 weeks (and optionally at 34 weeks) suppresses primary immune response to these occult bleeds, preventing maternal sensitisation. The passive anti-D does not cross in sufficient quantity to harm the fetus. Postnatal anti-D is additionally given within 72 hours of delivery if the newborn is Rh-positive.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.