Rh sensitization is the process by which the Rh-negative mother's immune system reacts to her fetus's Rh-positive blood. After fetal blood enters the mother's blood system, her immune system produces antibodies that destroy Rh-positive red blood cells.

Sensitization most commonly occurs during childbirth. About 90% of women who become sensitized do so at delivery, when maternal blood is most likely to mix with fetal blood.^{1, 2}

About 17% of Rh-negative women who do not receive Rh immune globulin will become sensitized during pregnancy or childbirth.¹ The minimum amount of blood mixing necessary to cause sensitization is not known. However, some women who become sensitized do so after becoming exposed to as little as 0.1 mL of Rh-positive blood.¹

Rh sensitization can almost always be prevented with an Rh immune globulin injection during pregnancy and after delivery. Of women given Rh immune globulin, less than 1% will become sensitized.³

Some women do not become sensitized even though they are exposed to large amounts of Rh-positive blood. The reason for this is not known.

Sensitization usually does not affect fetal health during the newly sensitized pregnancy. However, an Rh-positive fetus in a subsequent pregnancy can be severely affected.