What is the probability of an Rh- (Rhesus negative) pregnant woman having an Rh+ (Rhesus positive) baby?

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Probability of an Rh- Pregnant Woman Having an Rh+ Baby

The probability of an Rh-negative woman having an Rh-positive baby is approximately 60-70%, depending on the father's Rh status and genetic makeup. 1

Genetic Basis of Rh Status

The Rh factor is determined by a single gene with two alleles:

  • Rh positive (D) is dominant
  • Rh negative (d) is recessive 1

This means:

  • Rh-positive individuals can be either homozygous (DD) or heterozygous (Dd)
  • Rh-negative individuals must be homozygous recessive (dd) 1

Probability Calculations

The likelihood of having an Rh-positive baby depends on the father's Rh status:

  1. If the father is Rh-positive and homozygous (DD):

    • All offspring will be Rh-positive (Dd)
    • Probability of Rh+ baby = 100%

  2. If the father is Rh-positive and heterozygous (Dd):

    • 50% chance of passing the D allele
    • 50% chance of passing the d allele
    • Probability of Rh+ baby = 50%

  3. If the father is Rh-negative (dd):

    • All offspring will be Rh-negative (dd)
    • Probability of Rh+ baby = 0%

Since approximately 85% of Caucasians are Rh-positive (with varying rates in other ethnicities), and about half of these are heterozygous, the overall probability of an Rh-negative woman having an Rh-positive baby is around 60-70% 2.

Clinical Implications

This probability is clinically significant because:

  1. Risk of Alloimmunization: When an Rh-negative mother carries an Rh-positive fetus, there is risk for maternal alloimmunization if fetal blood enters maternal circulation 1

  2. Prophylaxis Requirements: Anti-D immunoglobulin (RhIg) prophylaxis is recommended for Rh-negative mothers carrying Rh-positive fetuses 3

  3. Testing Options: Fetal RHD genotyping using cell-free DNA from maternal plasma can determine fetal Rh status with approximately 94% accuracy 4

Management Recommendations

  • All Rh-negative pregnant women should receive anti-D immunoglobulin at 28 weeks' gestation when fetal blood type is unknown or known to be Rh-positive 3

  • When paternity is certain, testing the father's Rh status can help determine risk and eliminate unnecessary blood product administration 3

  • After delivery, anti-D immunoglobulin should be given within 72 hours to Rh-negative women who deliver Rh-positive infants 3

Important Considerations

  • Rh variants exist in approximately 2.2% of multiethnic populations, which can complicate serologic testing 5

  • The prevalence of Rh-negativity varies by ethnicity: 15% in Caucasians, 3-5% in Black Africans, and rare in Asians 2

  • Cell-free DNA testing for fetal RHD status is available and can help guide management decisions, though it is not 100% accurate 4

References

1
Guideline

Blood Group Systems in Clinical Practice

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

2
Research

Non-invasive prenatal diagnosis and determination of fetal Rh status.

Seminars in fetal & neonatal medicine, 2008

3
Research

Prevention of Rh alloimmunization.

Journal of obstetrics and gynaecology Canada : JOGC = Journal d'obstetrique et gynecologie du Canada : JOGC, 2003

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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