Can an Rh Positive Mother Become Rh Negative?
No, a mother who is truly Rh positive cannot become Rh negative—this is a fixed genetic trait determined at conception and does not change throughout life.
Understanding Rh Blood Type Genetics
- The Rh status (positive or negative) is determined by the presence or absence of the D antigen on red blood cells, which is genetically determined and permanent 1
- An individual is either born with the D antigen gene (Rh positive) or without it (Rh negative)—this cannot change over time 2
Why This Question Arises: Common Sources of Confusion
Laboratory Testing Variations
- Weak D (Du) phenotype can cause confusion: Some individuals have a weakly expressed D antigen that may test as "Du positive" on standard typing but could be misinterpreted 3
- In rare cases of massive fetomaternal hemorrhage, an Rh negative mother carrying an Rh positive fetus may have enough fetal Rh positive cells in her circulation to cause a false positive result if blood is typed close to the time of hemorrhage 3
- Laboratory errors or technical issues can occasionally produce incorrect results, but these represent testing problems, not actual changes in Rh status
Maternal-Fetal Cell Transfer
- Fetal red blood cells can enter maternal circulation (fetomaternal hemorrhage), occurring in 7% of first trimester pregnancies, 16% in second trimester, and 29% in third trimester 4
- These fetal cells display D-antigen from as early as 6 weeks gestation 1, 4
- However, the presence of fetal Rh positive cells in maternal circulation does not change the mother's own Rh status—it only creates a risk for alloimmunization 2
Clinical Implications
If a mother was previously typed as Rh positive and now appears Rh negative (or vice versa), this represents either:
- A laboratory error requiring repeat testing
- A rare variant phenotype (such as weak D) that needs specialized testing 3
- Sample mix-up or documentation error
The correct approach is to repeat the blood typing and resolve the discrepancy before making clinical decisions about RhIg prophylaxis 1