Why is a cord blood sample taken in an Rh (Rhesus)-negative pregnancy?

Why Cord Blood Sample is Taken in Rh-Negative Pregnancy

Cord blood sampling in Rh-negative pregnancies is performed primarily to determine the newborn's RhD status, which directly determines whether the mother requires postpartum RhD immune globulin (RhIg) administration within 72 hours to prevent alloimmunization and hemolytic disease in future pregnancies. 1, 2

Primary Purpose: Determining Need for Postpartum RhIg

The fundamental reason for cord blood testing is straightforward:

• If the infant is RhD-positive, the Rh-negative mother must receive RhIg within 72 hours postpartum to prevent sensitization 2
• If the infant is RhD-negative, no postpartum RhIg is needed since there is no risk of maternal alloimmunization 2
• Without RhIg prophylaxis, the rate of maternal alloimmunization after delivery of an RhD-positive infant is 12-13%, which is reduced to 1-2% with postpartum RhIg alone 2

What is Actually Tested on Cord Blood

The cord blood sample determines:

• Infant's RhD type (positive or negative) through serological testing 3, 4
• This confirms whether fetal RhD-positive red blood cells entered maternal circulation during delivery, creating sensitization risk 2

Important Clinical Context

The Two-Dose Protocol

• Most Rh-negative mothers receive antenatal RhIg at 28 weeks gestation 1, 2
• However, antenatal RhIg does NOT eliminate the need for postpartum RhIg if the infant is RhD-positive 2
• The two-dose protocol (28 weeks + postpartum) reduces alloimmunization from 1.6% to less than 0.1% 2

Timing is Critical

• RhIg must be administered within 72 hours of delivery to be effective 2
• This narrow window makes rapid cord blood testing essential for clinical decision-making 2

Evolution Away from Direct Antiglobulin Testing (DAT)

A critical caveat: While cord blood RhD typing remains standard, the direct antiglobulin test (DAT/Coombs test) on cord blood is no longer routinely recommended 5. This is because:

• Routine antenatal anti-RhD prophylaxis causes false-positive DAT results 5
• A positive cord DAT poorly predicts subsequent hyperbilirubinemia 5
• The focus should be on RhD typing, not antibody detection on cord cells 5

Alternative: Prenatal Cell-Free DNA Testing

Modern practice increasingly uses cell-free fetal DNA from maternal plasma to determine fetal RHD status:

• Sensitivity of 97.2% and specificity of 96.8% for detecting fetal RhD positivity 6
• Accuracy exceeds 99% when performed after 11 weeks gestation 7
• This allows targeted antenatal RhIg administration only to mothers carrying RhD-positive fetuses 3, 4

However, even with prenatal testing, cord blood confirmation remains important because:

• It provides definitive serological confirmation at delivery 3, 4
• It ensures appropriate postpartum RhIg administration 2
• Prenatal testing has a small false-positive rate that could lead to unnecessary RhIg 7

Clinical Algorithm

For every Rh-negative mother at delivery:

1. Obtain cord blood sample immediately after delivery 2
2. Perform RhD typing on cord blood (not DAT) 5
3. If infant is RhD-positive: Administer 300 μg RhIg to mother within 72 hours 2
4. If infant is RhD-negative: No postpartum RhIg needed 2
5. If father is confirmed RhD-negative: RhIg not needed regardless of cord blood result 2

Common Pitfalls to Avoid

• Never assume the antenatal dose at 28 weeks is sufficient - postpartum RhIg is still required if the infant is RhD-positive 2
• Do not rely on cord blood DAT for clinical decision-making about hyperbilirubinemia risk 5
• Do not delay beyond 72 hours - efficacy of RhIg decreases significantly after this window 2
• Remember that fetal RBCs display RhD antigens from 6 weeks gestation, making sensitization possible even in early pregnancy events 1