Rh Sensitization During Pregnancy

Rh sensitization can occur when a person with Reference Rh-negative blood Opens New Window is exposed to Rh-positive blood. About 90% of women who become sensitized do so during childbirth, when their blood mixes with the Rh-positive blood of their fetus.^{Reference 1} After being exposed, a mother's Reference immune system Opens New Window produces Reference antibodies Opens New Window against Rh-positive red blood cells.

The minimum amount of blood mixing that causes sensitization is not known. But many women become sensitized during pregnancy or childbirth after being exposed to as little as 0.1 mL of Rh-positive fetal blood.^{Reference 1} Fortunately, Rh sensitization can almost always be prevented with the Reference Rh immune globulin injection.

When an Rh-negative person's immune system is first exposed to Rh-positive blood, it takes several weeks to develop immunoglobulin M, or IgM, antibodies. IgM antibodies are too large to cross the Reference placenta Opens New Window. So the Rh-positive fetus that first triggers maternal sensitization is usually not harmed.

A previously Rh-sensitized immune system rapidly reacts to Rh-positive blood, as during a second pregnancy with an Rh-positive fetus. Usually within hours of Rh-positive blood exposure, smaller immunoglobulin G, or IgG, antibodies are formed. IgG antibodies can cross the placenta and destroy fetal red blood cells. This causes Reference Rh disease Opens New Window, which is dangerous for the fetus.

Some Rh-negative people never become sensitized, even after exposure to large amounts of Rh-positive blood. The reason for this is not known.

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