The only time Rh sensitization occurs is when you have an Rh- mom and a Rh+ fetus. The Rh+ blood from the baby comes in contact with the mother’s Rh- blood. This mixing of blood occurs at birth, but can also happen during a miscarriage, amniocentesis, or when there is trauma to mom’s abdomen.
The mom looks at the Rh+ blood as a foreign body [antigen] and then produces antibodies to fight it. The 1st offspring is not affected by the antibodies, because if you have your baby today, you don’t have antibodies today. This is what we are trying to prevent. If the mom makes the antibodies, you can never get rid of it, and it gets worse and worse with each pregnancy.
If a Rh- sensitized mom [meaning she already has the antibodies] gets pregnant again, she has her little army ready to attack the Rh+ blood from the new fetus. Her antibodies enter through the placenta and hemolysis occurs [breakdown of red blood cells in the fetus.] The baby will not grow, and will try to compensate by releasing immature red blood cells, this is known as erythroblastosis fetalis.
It will cause anemia, hypoxia, hyperbilirubinemia, congestive heart failure, neurologic damage, and hydrops fetalis in the fetus. Poor baby :(