What dose of Anti-D (Rho(D) immune globulin) should be administered to an RhD negative mother who received Anti-D at 24 weeks of gestation and has delivered an RhD positive baby?

Postpartum Anti-D Dosing for Rh-Negative Mother

Give 300 micrograms (option A) of Anti-D immune globulin within 72 hours of delivery, as this is the standard postpartum dose recommended by ACOG and international guidelines for an Rh-negative mother who delivered an Rh-positive baby. 1, 2

Standard Postpartum Protocol

The mother should receive the full standard postpartum dose despite having received antenatal prophylaxis at 24 weeks, because:

• The antenatal dose at 28 weeks (or in this case 24 weeks) does NOT replace the postpartum dose - these are two separate components of the prophylaxis protocol designed to work together 1
• The two-dose protocol (antenatal + postpartum) reduces RhD alloimmunization from approximately 1.8% to between 0.1% and 0.2%, compared to postpartum dosing alone which only reduces it from 13-17% to 1-2% 1
• Approximately 90% of fetomaternal hemorrhage occurs during delivery itself, making the postpartum dose critical regardless of prior antenatal prophylaxis 3

Dosing Specifications

• Standard dose: 300 micrograms (1500 IU) IM or IV within 72 hours of delivery 2
• Alternative acceptable regimen: 120 micrograms may be used if that is the available preparation, though testing for fetomaternal hemorrhage >6 mL fetal RBCs would then be required 2
• If the 72-hour window is missed, Anti-D should still be administered up to 28 days postpartum, as delayed administration provides some protection and is preferable to none 1, 2

Critical Additional Considerations

Screen for excessive fetomaternal hemorrhage in this patient, particularly since she received antenatal Anti-D earlier than the standard 28 weeks (given at 24 weeks instead):

• If fetomaternal hemorrhage exceeds 15 mL of fetal red blood cells (approximately 30 mL of fetal blood), additional Anti-D is required at 10 micrograms per 0.5 mL of fetal red blood cells 2
• Quantitative testing (such as Kleihauer-Betke or flow cytometry) should be considered to determine if additional doses are needed 2, 4
• The standard 300 microgram dose covers up to 15 mL of fetal RBCs; larger hemorrhages require supplemental dosing 2

Common Pitfalls to Avoid

• Do not withhold the postpartum dose assuming the antenatal dose at 24 weeks provides adequate coverage - both doses are required for optimal protection 1
• Do not delay beyond 72 hours - efficacy decreases with time, though late administration is still beneficial up to 28 days 1, 2
• Do not forget to confirm the baby is actually Rh-positive before administering, though if testing is unavailable or delayed, administer Anti-D rather than risk sensitization 1