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Fetomaternal hemorrhage (FMH), which can occur during pregnancy or delivery, triggers the Rh-negative mother's immune system to develop antibodies against the antigens in her baby's Rh-positive blood. This is called Rh-sensitization or alloimmunization. While this is the most common cause for Rh-sensitization, there are other ways in which sensitization can occur. For instance, Rh-sensitization occurs when someone with Rh-negative blood receives a transfusion of Rh-positive blood or blood components, such as platelets.
While the placenta-cell layer (trophoblast) usually prevents the fetus's blood from entering the mother's bloodstream, certain conditions can create FMH. The resulting Rh-sensitization can occur during any of these events:
- Delivery
- Miscarriage
- Amniocentesis
- Chorionic villus sampling
- Placental abruption
- Manual extraction of placenta
- External version
- Childbirth
- Abortion
Typically, only a small number of antibodies are produced in the first pregnancy. During delivery, however, the mother's immune system becomes sensitized to the Rh factor. In subsequent pregnancies, the mother produces a greater number of antibodies, which cross the placenta to destroy the fetal red blood cells and create significant health risks for the baby. With each subsequent Rh-positive pregnancy, the mother's antibody reaction is faster, with potentially more severe consequences.
The mother needs to receive anti-D IgG treatment during the first pregnancy and immediately following delivery to thwart the danger that can occur with future pregnancies with an Rh-positive fetus.
- Rh-negative mothers have a 1%–2% risk of being sensitized during the last trimester of pregnancy
—At delivery, the Rh-negative mother has a 10%–15% risk of Rh-sensitization
- An Rh-negative mother has about a 60% chance of having an Rh-positive baby if the father is Rh-positive
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