RhIg Dosing at 6 Weeks Gestation
No, 150 mcg is not the recommended dose of RhIg at 6 weeks gestation—the correct dose is 50 mcg (or 120 mcg if 50 mcg is unavailable), not 150 mcg. 1
Correct Dosing for Early First Trimester
For pregnancy complications before 12 weeks gestation, including at 6 weeks, the recommended dose is 50 μg (micrograms) of RhIg administered within 72 hours. 1
If the 50 μg dose is unavailable, the standard 300 μg dose should be used as an alternative. 1
The 120 μg dose is cited as the minimum acceptable dose for first trimester events in some guidelines, representing the lowest currently available dose in certain formulations. 2
Why RhIg Is Necessary at 6 Weeks
The rationale for administering RhIg even this early in pregnancy is critical to understand:
Fetal red blood cells display RhD antigens from as early as 6 weeks of gestation, making maternal sensitization physiologically possible even in very early pregnancy. 1
The risk of alloimmunization can occur with any pregnancy loss or bleeding event, with potentially devastating consequences for future pregnancies including hemolytic disease of the fetus/newborn, fetal hydrops, and stillbirth. 1
While some international organizations have suggested RhIg may be unnecessary before 12 weeks based on logistical considerations, these recommendations are not based on evidence of safety—existing data do not convincingly demonstrate the safety of withholding RhIg for first-trimester losses. 1
Clinical Application
For an Rh-negative patient at 6 weeks gestation experiencing:
- Miscarriage/pregnancy loss: Administer 50 μg RhIg within 72 hours 1
- Threatened abortion with heavy bleeding or abdominal pain: Administer 50 μg RhIg within 72 hours 1
- Ectopic pregnancy: Administer minimum 120 μg RhIg before 12 weeks 2
Common Pitfall to Avoid
Do not withhold RhIg based on early gestational age alone—this is not supported by convincing evidence and represents a critical error given that sensitization is biologically possible from 6 weeks onward. 1