What are the management options for a 16-week pregnant woman with B negative blood type and a positive antibody screen?

Management of B-Negative Pregnant Woman with Positive Antibody Screen at 16 Weeks

A pregnant woman at 16 weeks gestation with B-negative blood type and a positive antibody screen requires immediate administration of Rh immune globulin (RhIg) to prevent alloimmunization, along with further antibody identification and monitoring throughout pregnancy.

Understanding the Significance of a Positive Antibody Screen

• A positive antibody screen in a B-negative pregnant woman indicates potential alloimmunization, which could lead to hemolytic disease of the fetus and newborn (HDFN) if left untreated 1.
• Alloimmunization occurs when maternal antibodies cross the placenta and attack fetal red blood cells that express antigens inherited from the father, causing fetal anemia 2.
• While most attention is given to Rh(D) incompatibility, other blood group antibodies can also cause significant HDFN 3.

Immediate Management Steps

1. Administer Rh immune globulin (RhIg)

   • Give a full dose of RhIg (1500 IU; 300 mcg) intramuscularly as soon as possible 4.
   • RhIg should be administered following threatened abortion at any stage of gestation with continuation of pregnancy 4.
   • The medication should be given intramuscularly, preferably in the deltoid muscle of the upper arm or lateral thigh muscle (avoid gluteal region due to risk of sciatic nerve injury) 4.

2. Identify the specific antibody

   • Determine which antibody is causing the positive screen, as this could be anti-D or other clinically significant antibodies (anti-Kell, anti-Duffy, etc.) 1.
   • The specific antibody identity will guide further management and risk assessment 2.

Ongoing Management

1. Serial antibody titers

   • Monitor antibody titers every 2-4 weeks, depending on the specific antibody identified and its clinical significance 1.
   • Rising titers may indicate increasing risk of fetal hemolysis 2.

2. Middle cerebral artery (MCA) Doppler ultrasound

   • Begin MCA Doppler assessments at 18-20 weeks if clinically significant antibodies are identified 1.
   • Increased MCA peak systolic velocity suggests fetal anemia requiring intervention 1.

3. Detailed ultrasound examination

   • Perform detailed anatomical survey to assess for signs of fetal hydrops or other complications 1.
   • Monitor fetal growth regularly throughout pregnancy 1.

4. Additional RhIg administration

   • Administer another dose of RhIg at 28 weeks' gestation 4.
   • "For antenatal prophylaxis, one full dose syringe of RhIg (1500 IU; 300 mcg) is administered at approximately 28 weeks' gestation. This must be followed by another full dose (1500 IU; 300 mcg), preferably within 72 hours following delivery, if the infant is Rh positive" 4.

5. Fetal blood sampling consideration

   • Consider invasive testing (amniocentesis or cordocentesis) if there are signs of fetal anemia or hydrops 1.
   • These procedures carry small risks of pregnancy loss or preterm labor 1.

Special Considerations

• Potential for severe HDFN: Even non-Rh antibodies can cause significant hemolysis requiring intervention 3.
• Intravenous immunoglobulin (IVIG): Consider IVIG in cases of severe alloimmunization to prevent or delay the onset of severe fetal anemia 1.
• Timing of delivery: May need to consider early delivery if severe anemia develops later in pregnancy 1.

Common Pitfalls to Avoid

• Focusing only on Rh(D) status: While Rh(D) incompatibility is most common, other antibodies can cause significant HDFN and should not be overlooked 3.
• Inadequate follow-up: Failure to monitor antibody titers and fetal well-being can result in missed opportunities for intervention 2.
• Delayed RhIg administration: RhIg is most effective when given promptly, though it still provides some protection when given beyond the 72-hour window 4.
• Assuming all antibody screens in Rh-positive women are clinically insignificant: While less common, clinically significant antibodies can occur in Rh-positive women as well 5.

Post-Delivery Management

• Administer RhIg within 72 hours after delivery if the baby is Rh-positive 4.
• If a large fetomaternal hemorrhage is suspected (>15 mL of fetal red blood cells), perform a Kleihauer-Betke test to determine appropriate RhIg dosage 4.
• Monitor the newborn for signs of hemolysis, anemia, and hyperbilirubinemia 2.