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Alloimmunization and Red Cell Antibodies – Time to be Concerned?


The care of patients with sensitization to antigens other than D, that are known to cause hemolytic disease, should be the same as that for patients with D alloimmunization. ACOG has provided updated guidance (March 2018) that includes the following recommendations:

In a center with trained personnel and when the fetus is at an appropriate gestational age, Doppler measurement of peak systolic velocity in the fetal middle cerebral artery is an appropriate noninvasive means to monitor pregnancies complicated by red cell alloimmunization

The initial management of a pregnancy involving an alloimmunized patient is determination of the paternal erythrocyte antigen status

Serial titers are not useful for monitoring fetal status when the mother has had a previously affected fetus or neonate

Antibody titers are not appropriate for monitoring Kell-sensitized patients because Kell antibodies do not correlate with fetal status

Anti-D immune globulin is indicated only in Rh-negative women who are not previously sensitized to D

In addition, The AABB working group addressed the work up and management of ‘weak D’ (formerly Du) in their joint statement that included representatives from ACOG, CAP, American Red Cross, Armed Services Blood Program

Persons with a weak D type 1, 2 or 3 can be managed safely as Rh-positive and such women, if pregnant, do not require Rh immune globulin. For more than 50 years, the recommended practice in the United States has been to Rh type patients using laboratory methods that interpret weak D phenotypes as Rh-negative. The intent of this practice has been to protect Rh-negative persons, particularly Rh-negative women of childbearing potential, from inadvertent exposure and alloimmunization to Rh-positive red blood cells. RHD genotyping methods are now available that can identify those persons with a weak D phenotype who can be managed as Rh-positive (weak D types 1, 2 or 3).

ACOG has responded to the AABB working group above and advises that for weak D (previously known as Du)

An attractive solution to this problem is to perform molecular genetic RHD typing in weak D phenotype individuals as suggested by the Work Group on RHD Genotyping.

Currently, there is a lack of comprehensive cost-benefit analysis for this clinical approach. Clinicians are advised to administer Rh D immune globulin to patients with weak D blood type in appropriate clinical situations, by the same rationale as that for Rh D typing blood donors, until further scientific and economic studies are available. 


Antepartum or intrapartum fetal-maternal bleeding may stimulate an immune response in the mother when any fetal blood group factor inherited from the father is not possessed by the mother. Maternal antibodies may form (alloimmunization) and there may be transplacental passage of these antibodies into the fetal circulation. This transplacental passage of antibodies into the fetal circulation may lead to hemolytic disease in the fetus and newborn. The risk of hemolytic disease is determined by the degree of antigenicity and the amount and type of antibody involved.  The AABB recommends that patients should undergo repeat screening before receiving anti-D immune globulin at 28 weeks, postpartum and at the time of any event in pregnancy.


  • Five major antigens can be identified for the Rh (C,D,E) blood group system, these are:
    • C,c,D,E,e
    • In addition to the 5 antigens of the Rh system, there are more than 30 red cell antigen groups that have been described and many of these can can cause hemolytic disease in the newborn and fetus
  • Most cases of Rh alloimmunization causing hemolytic disease are the result of incompatibility with respect to the D antigen
  • The red cell antigen groups and their association with hemolytic disease:
Blood Group System Antigen Hemolytic Disease Severity
 Lewis  No proven risk
 I  No proven risk
 Xg Xga  Mild
Lutheran Lua


 Kell K  Severe including hydrops






 Rh E and c (non-D) c




 Mild to severe (c and E can

lead to hydrops); e and C are rare

 Duffy Fya  Mild to severe including hydrops
Fyb  Not a known cause of HDN
Fy3  Mild
 Kidd Jka  Mild to severe






 Mild to severe
Mia  Moderate
N  Mild
 MSSs Mta  Moderate





 P PP1pk(Tja)  Mild to severe
 Diego D1a


 Mild to severe
 Xg Xga  Mild
 Lutheran Lua



Note: Mild only, with no risk to advance to a higher risk category, can be treated with routine obstetric care.  Any risk of moderate or above requires referral for fetal assessment

Learn More – Primary Sources:

ACOG Practice Bulletin No. 192: Management of Alloimmunization During Pregnancy

ACOG Practice Bulletin No. 181: Prevention of Rh D Alloimmunization

Dean L (NCBI): Blood Groups and Red Cell Antigens

AABB: Joint Statement on Phasing-In RHD Genotyping for Pregnant Women and Other Females of Childbearing Potential with a Serologic Weak D Phenotype

Locate a Maternal Fetal Medicine Specialist:

When to Administer Anti-D Immune Globulin to Prevent Rh D Alloimmunization


Rh D alloimmunization can be prevented with the administration of anti-D immune globulin.  ACOG recommends administration of anti-D immune globulin to unsensitized Rh D-negative women as follows

Level A Recommendation (good and consistent science)

  • 28 weeks (reduces risk from 2% to 0.2%)
  • Within 72 hours of birth, if infant is Rh D positive and patient is not sensitized

Level B Recommendation (limited or inconsistent science)

  • All invasive diagnostic procedures such as CVS and amniocentesis if fetus may be Rh D positive

Level C Recommendation (consensus/expert opinion)

ACOG language in (” “)

  • External cephalic version, regardless of success (“indicated”)
  • Evacuation of suspected molar pregnancy (“reasonable to administer”)
  • First trimester miscarriage
    • Spontaneous (1.5-2% risk – “should be considered”)
    • Instrumentation (4-5% risk – “should be given”)
  • Termination of pregnancy, either medical or surgical (“should be given”)
  • Ectopic pregnancy (“recommended”)
  • Antenatal hemorrhage >20 weeks gestation (“recommended”)
  • Abdominal trauma (“should be administered”)
  • Fetal death in the 2nd or 3rd trimester (“should be administered”)


All pregnant women are currently tested at the time of the first prenatal visit for ABO blood group and Rh D type and screening for red cell antibodies. The potential volume of fetal–maternal hemorrhage that can cause Rh D alloimmunization is extremely variable and ranges from 0.1 mL to 30 mL.  Data is limited when it comes to many of the potential sensitizing events and therefore some of the recommendations are based on consensus and expert opinion.


  • Multiple consensus guidelines worldwide recommend
    • Patients should undergo repeat Rh D antibody screening before receiving anti-D immune globulin at 28 weeks and postpartum
  • Providers should be alert to any other events in pregnancy that may increase risk for fetal-maternal hemorrhage
  • ACOG advises that for weak D (previously known as Du)

Clinicians are advised to administer Rh D immune globulin to patients with weak D blood type in appropriate clinical situations, by the same rationale as that for Rh D typing blood donors, until further scientific and economic studies are available.

  • Anti-D immune globulin is purified and tested for viral infections, and therefore risk of viral infection is exceedingly low
    • Manufactured without mercury-containing thimerosal since 2001
  • Failures may still occur due to
    • Not administering in 3rd trimester
    • Insufficient dosage or not administering within 72 hours after a known sensitizing pregnancy event or following birth
    • Unrecognized fetal–maternal hemorrhage
  • Note: There is discussion in the CDC recommendations that receipt of antibody-containing blood products such as intravenous immune globulin may interfere with rubella (and MMR) vaccine and seroconversion. However, the CDC states

Previous administration of human anti-Rho(D) immune globulin (RhoGam) does not generally interfere with an immune response to rubella vaccine and is not a contraindication to postpartum vaccination. However, women who have received anti-Rho immune globulin should be serologically tested 6–8 weeks after vaccination to ensure that seroconversion has occurred.

Assessing Fetal-Maternal Hemorrhage Volume

  • Screening
    • Rosette fetal red blood cell assay to detect greater than 2 mL of fetal whole blood in maternal circulation
  • Quantification tests are appropriate if rosette test is positive
    • Kleihauer-Betke (inexpensive, but lacks standardization and precision)
      • Volume of fetal bleed: % fetal cells x maternal blood volume
      • Maternal blood volume: 70 mL/kg x weight (kg) (assume 5,000 mL if maternal information is unknown)
    • Flow cytometry (more accurate, but limited by cost and availability)

Dosing (ACOG)

  • Screen all Rh D-negative women with Rh D-positive infants for fetal-maternal hemorrhage volume
  • Standard 300-microgram dose of anti-D immune globulin covers 30 mL of Rh D-positive fetal whole blood (or 15 mL of fetal red blood cells)
  • Exposures to <30 mL of Rh D-positive fetal whole blood
    • 28 weeks: 300 micrograms
    • After birth (within 72 hours): 300 micrograms
  • Exposures to >30 mL of Rh D-positive fetal whole blood
    • Assess volume of fetal-maternal hemorrhage to guide dosing
    • Can use up to eight full vials at one time
    • IM: Separate sites every 12 hours until the desired dosage has been reached
    • IV: Rh immune globulin is also available


ACOG Practice Bulletin 181: Prevention of Rh D Alloimmunization

ACOG Practice Bulletin 192: Management of Alloimmunization During Pregnancy

CDC: Epidemiology and Prevention of Vaccine-Preventable Diseases – Rubella

Locate a Maternal Fetal Medicine Specialist

Maternal Fetal Medicine Specialist Locator-SMFM