Why is the D antigen (Rh factor) important during pregnancy?

D antigen is a protein with many parts, found on the surface of red blood cells. The D antigen is also known as the "Rh factor," and it tells your blood type. People are either D (Rh) negative or D (Rh) positive. If your blood type is D negative, your red blood cells do not have the D antigen.

For some people, the D antigen may show up only weakly. Red blood cells from these people are known as "D variant." There are two types of D variant antigens:

• "Weak D" means all parts of the D antigen are present but weakly found on the red cell surface.
• "Partial D" means that one or more parts of the D antigen are missing.

Mothers who are "partial D" should be considered D (Rh) negative. These mothers could build up antibodies to the part of the "D" antigen they lack. If their baby is D (Rh) positive, the antibodies could destroy the baby's red blood cells. This leads to hemolytic disease of the fetus and newborn, or "HDFN." These mothers will need treatment during pregnancy to prevent HDFN.

Mothers who are "weak D" can be considered Rh positive, so they do not need this treatment.