A Rh-negative woman at 28 weeks gestation receives antenatal anti-D prophylaxis. At delivery, her baby is Rh-positive. The Kleihauer-Betke test shows 0.2% fetal cells in maternal circulation. Maternal blood volume is 4500 mL, haematocrit 33%. Calculate the volume of fetomaternal haemorrhage (FMH) in mL of fetal red cells and the appropriate anti-D dose.

• A FMH = 4.5 mL fetal RBCs; 1 vial (300 mcg) anti-D is sufficient
• B FMH = 9.0 mL fetal RBCs; 1 vial (300 mcg) anti-D is sufficient
• C FMH = 2.97 mL fetal RBCs; 2 vials (600 mcg) are required
• D FMH = 2.97 mL fetal RBCs; 1 vial (300 mcg) anti-D is sufficient ✓

Explanation

FMH calculation: Fetal RBC volume = % fetal cells × maternal blood volume × maternal haematocrit = 0.002 × 4500 mL × 0.33 = 2.97 mL of fetal red cells. Each standard 300 mcg vial of anti-D covers 15 mL of fetal red cells (30 mL of whole fetal blood). Since 2.97 mL < 15 mL, one vial (300 mcg) is sufficient. No rounding up to additional vials is needed here as the volume is well below the single-vial threshold. Additional vials are needed only when FMH exceeds 15 mL of fetal red cells.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.