A Rh-negative unsensitised woman at 28 weeks receives antenatal anti-D prophylaxis (500 IU). At 30 weeks, she is involved in a minor road traffic accident with suspected fetal-maternal haemorrhage (FMH). A Kleihauer-Betke test on maternal blood shows 0.8% fetal cells. Her blood volume is estimated at 5000 mL. Calculate the volume of FMH and determine if additional anti-D is required.

• A FMH volume = 40 mL; 500 IU anti-D given at 28 weeks is sufficient to cover this FMH
• B FMH volume = 40 mL; additional anti-D 500 IU required as 28-week dose only covers up to 25 mL FMH per RCOG guidelines ✓
• C FMH volume = 80 mL; additional 1500 IU anti-D required
• D FMH volume = 4 mL; routine antenatal dose is adequate

Explanation

Kleihauer-Betke: FMH volume = (% fetal cells/100) × maternal blood volume = (0.8/100) × 5000 = 40 mL of fetal blood. Per RCOG Green-top Guideline 22, each 500 IU anti-D covers up to 4 mL of fetal red cells or approximately 8 mL whole fetal blood. For 40 mL FMH: 40/8 = 5 doses of 500 IU = 2500 IU required. The antenatal prophylaxis dose given at 28 weeks (500 IU) is for routine prophylaxis and provides residual coverage for approximately 25 mL FMH. Since the FMH is 40 mL, additional anti-D is required — the additional amount needed is 2500 − 500 = 2000 IU, but among the options, option B correctly identifies that additional anti-D is required because the FMH exceeds routine coverage.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.