What is the recommended dose of Rhogam (Rho(D) immune globulin) for vaginal bleeding during pregnancy in an Rh-negative mother?

Rhogam Dosing for Vaginal Bleeding During Pregnancy

For Rh-negative mothers experiencing vaginal bleeding during pregnancy, a full dose of 300 mcg (1500 IU) of Rho(D) immune globulin should be administered intramuscularly, preferably within 72 hours of the bleeding episode. 1, 2

Dosing Recommendations Based on Gestational Age

First Trimester (<13 weeks)

• Threatened abortion with continuing pregnancy:
  • Full dose (300 mcg) recommended, especially with:
    • Heavy bleeding
    • Associated abdominal pain
    • Event occurring near 12 weeks' gestation 1
  • Although historically a microdose (50 mcg) was considered adequate for early pregnancy events before 12 weeks, current guidelines favor the full dose 1, 2

Second and Third Trimester (≥13 weeks)

• Always administer full dose (300 mcg) for any vaginal bleeding 2
• For routine prophylaxis, administer at 28 weeks' gestation and again within 72 hours after delivery if infant is Rh-positive 2

Administration Guidelines

• Route of administration: Intramuscular only, NEVER intravenously 2
• Preferred injection site: Deltoid muscle or lateral thigh muscle 2
• Avoid: Gluteal region (risk of sciatic nerve injury) 2

Special Considerations

Large Fetomaternal Hemorrhage

• Standard dose (300 mcg) provides protection for up to 15 mL of fetal red blood cells 2
• If large fetomaternal hemorrhage (>15 mL RBCs) is suspected:
  1. Perform quantitative testing (e.g., Kleihauer-Betke test)
  2. Calculate additional doses needed: divide RBC volume by 15 mL
  3. If calculation results in a fraction, administer the next higher whole number of syringes 2

Timing of Administration

• Optimal timing: Within 72 hours of bleeding episode 2
• If not given within 72 hours, should still be administered up to 28 days after the event, though with potentially decreased effectiveness 3

Evidence and Rationale

• Fetal red blood cells display RhD antigens from as early as 6 weeks gestation, creating potential for maternal sensitization even in early pregnancy 1
• Studies show fetomaternal hemorrhage occurs in 48% of threatened abortions, emphasizing the importance of RhoGAM administration 1
• While Rh immunization from first trimester threatened abortion is described as "exceedingly rare" by ACOG, the theoretical risk and minimal harm from administration generally favor its use 4, 1

Important Caveats

• Only unsensitized Rh-negative women (those with negative antibody screen) are eligible for RhoGAM 1
• Rh-positive women do not require RhoGAM 1
• Women with "weak D" (Du-positive) should not receive anti-D 3
• Obesity may affect efficacy of standard dosing, with some cases of alloimmunization reported in obese patients despite appropriate prophylaxis 5

The evidence supporting RhoGAM use in first trimester bleeding is based primarily on expert opinion and extrapolation from experience with fetomaternal hemorrhage in late pregnancy, rather than robust clinical trials 6. However, given the serious consequences of Rh sensitization and the minimal risks of administration, prophylaxis is strongly recommended.