Why doesn't HDN usually occur during a first pregnancy of an Rh- mother and Rh+ fetus?

• A. The maternal immune system is always suppressed during the first pregnancy
• B. Fetal cells do not enter the maternal bloodstream during delivery
• C. Fetal cells normally enter maternal bloodstream during delivery, so the immune system is not stimulated to produce Rh antibodies
• D. Rh antigens are not expressed in the fetus during the first pregnancy

Answer: (C)

The key idea is that HDN from Rh incompatibility depends on maternal sensitization to the Rh(D) antigen and how long it takes for antibodies to form and cross the placenta.

During delivery, fetal red blood cells can enter the mother’s bloodstream, exposing her immune system to the Rh(D) antigen. This exposure can trigger an immune response and the production of anti-D antibodies. However, these antibodies are not produced instantly; it takes time for the maternal immune system to generate enough IgG anti-D that can cross the placenta and hemolyze fetal red blood cells. If the fetus in the first pregnancy is Rh+, there isn’t enough maternal antibody yet to cause significant HDN. In later pregnancies, those antibodies can be present in sufficient amounts to affect the Rh+ fetus.

This is also why Rh prophylaxis with Rh immune globulin is used: it binds any Rh(D) antigens in the mother’s blood after exposure and prevents her immune system from becoming sensitized in the first place.