Rho D ALLOIMMUNIZATION

About 17% of Rh-negative women who don't receive anti-D immune globulin ("RhoGAM") will become alloimmunized.

Alloimmunization is caused by feto-maternal hemorrhage which occurs at delivery (90% of cases) as well as at certain prenatal events:

• Induceed or spontaneous abortion (including for molar pregnancy)
• Ectopic pregnancy
• External cephalic version
• Threatened abortion (rare; controversial whether RhoGAM should be given)
• 2nd- and 3rd-trimester bleeding
• Abdominal trauma
• Amniocentesis or chorionic villus sampling

Standard is to give RhoGAM if:

• To Rh-negative women at 28-29wks or
• Within 72h after delivery, or
• Earlier than 28-29wks if any of the above risky events occur.

RhoGAM is unnecessary if:

• Father is known to be Rh-negative
• Anti-Rh-D Ab are present (detected on the "Antibody screen")

If delivery hasn't occurred within 12wks after the first dose, a 2nd dose is advised

Dosing:

• Standard = 300ug IM
• During 1st trimester, only need 50ug IM
• If excessive feto-maternal hemorrhage (> 30mL) occurs, need more (can estimate with Kleihauer-Betke or "rosette" test)--must check in all women hwo deliver Rho D-positive infants!

(Source: ACOG Practice Bulletin #4, 5/94)