Screening and Treating Maternal/Fetal Rh Incompatibility and Alloimmunization
Posted by dkwinter

See also: Routine prenatal care

 

Risk is that mother may form antibodies to Rh antigen on fetus' RBCs, which cross the placenta and destroy fetal RBCs potentially resulting in severe anemia (erythroblastosis fetalis), high output cardiac failure, hydrops fetalis, and possibly fetal demise.

Prenatal cerebral blood flow via ultrasound is one way to ascertain fetal cardiac output.

Provide RhD blood typing and antibody testing:

  • at first prenatal visit in all cases
  • repeat RhD antibody testing for all Rh-negative women at 24-28 gestation unless the biological father is known and known to be RhD-negative

If antibody-negative

  • give RhD immunoglobulin at 28 weeks (do not wait for repeated antibody testing results according to ACOG)
  • Administration of anti-D gamma globulin (RhoGAM) prevents isoimmunization by binding the D antigens on fetal blood in the maternal circulation, thereby preventing the mother's immune system from reacting to them.

Give another dose if not alloimmunized and has a procedure or bleeding event (spontaneous abortion, threatened abortion > 12 weeks gestation unless significant bleeding, termination of pregnancy, ectopic pregnancy, amniocentesis, chorionic villus sampling, fetal blood sampling, D&C for hydatidiform mole, blunt force trauma, bleeding to to placenta previa or abruption, external cephalic version, intrauterine fetal death) (and don't wait for antibody testing results if not available):

  • give 50 mcg RhD immunoglobulin (note: vials contain 300 mcg RhD immunoglobulin)
    • standard dose for first trimester (up until 12 weeks)
  • give 300 mcg RhD immunoglobulin if beyond 12 weeks
    • provides protection against 15 mL of Rh+ RBCs (30 mL of Rh+ fetal whole blood)
    • there is NO adverse consequence of giving 300 mcg dose during first trimester
  • protection lasts 21-24 days

After delivery of an Rh-positive child (determined by cord blood) to an Rh-negative mother, additional screening should be done to determine if additional RhD immunoglobulin is required (Kleihauer–Betke Test or Rosette Test

 

If a mother is not sensitized (or has a VERY weak titer), anti-D immunoglobulin is indicated.
If a mother is already sensitized (antibody titers >=1:6), administration of RhoGAM is not helpful and close fetal monitoring for hemolytic disease is required.
The critical antibody titers that put a fetus at risk for hemolytic disease are usually between 1:8 and 1:32 (1:16 is frequently cited as critical).