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A recent CMAJ Practice article1 on mod­ ern Rhesus (Rh) typing in transfusion and pregnancy and its associated cor­ respondence2 prompted a productive discussion on safe recommendations for pregnant patients with a weak D type 4.0 allele, originally described in 2000.3 The differing approaches1,2 represent the personal views of the respective authors. Based on ou r review of 20 years’ worth of literature on this spe­ cialized topic, we have agreed on the following 5 statements: 1. No published case reports have docu­ mented adverse clinical effects, such as hemolysis, among pregnant people with weak D type 4.0 caused by an allo­ or auto­anti–D. 2. Similarly, no published case reports have documented adverse clinical effects, such as anemia or jaundice, among fetuses or newborns caused by such a mother’s allo­ or auto­anti–D. 3. No published evidence has shown that Rh immunoglobulin (RhIg) is clinically effective in an individual with weak D type 4.0 (e.g., for preventing anti­D for­ mation); RhIg can cause a positive direct antiglobulin test, which does not imply clinical harm. 4. The weak D type 4.0 phenotype may be associated with a proportionately larger number of anti­D than most other weak D types.4–6 The nature of these antibodies has not been well characterized (i.e., allo­ v. auto­ antibody).6 A fraction of all people with a weak D type 4.0 are routinely typed as normal RhD­positive and do not receive RhIg.3,7,8 5. The decision of whether or not to use RhIg or RhD­negative transfusion in such mothers should be based on national guidelines.9 Both approaches have been adopted by expert groups2,6,8–10 and are considered safe. The decision may still depend on an individual patient’s cir­ cumstances.11 If providers are unsure, consultation with a transfusion medi­ cine physician or perinatal immuno­ hematology reference laboratory is recommended.1,10

What constitutes the most cautious approach for a pregnant person with weak D type 4.0?