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When a pregnant woman's blood is Rh-negative, it is still likely that her baby will have Rh-positive blood. At some point during the pregnancy (usually labor), the baby's blood "leaks" into the mother's body, causing her to produce antibodies against the Rh-positive blood cells. This results in a condition called "Rh sensitization," wherein the antibodies can attack the baby's red blood cells and cause medical complications. The first pregnancy is the least complicated, but if the mother chooses to have any additional children, the complications from Rh sensitization can increase dramatically.

Upon finding out that you are pregnant, your doctor will perform a routine blood test to determine your blood type. Approximately 10% of the population has Rh-negative blood, so this is a relatively normal complication among pregnant women.

Rh sensitization is completely preventable with an injection called Rh Immunoglobulin. Unless you miscarry or have an abortion, you receive your first shot at about 7 months gestation. The shot lasts for 12 weeks and basically keeps the mother's immune system from destroying the Rh-positive fetal cells, preventing the creation of the antibodies and, in turn, the Rh sensitization. If the baby is found to be Rh-positive at birth, the mother is given another shot to prevent any postpartum complications.

Without the immunization, Rh sensitization can cause a range of complications for the newborn baby, including anemia, jaundice, and even stillbirth. Some babies require blood transfusions in order to stabilize the red blood cells and cleanse the body of the attacking antibodies. With each pregnancy, the mother's immune system creates a larger defense against the Rh-positive blood cells, thus creating more problems for the fetus. It is absolutely necessary to receive this shot from your first pregnancy on, as the immunoglobulin needs to meet your blood cells within 72 hours of the Rh+ blood to be successful.

Hyphenated says: The brand name of the injection is Rhogam.
fallensparks says: It may be worth clarifying, the injection you give is anti-RhD IgG. It's actually the same antibody that you are trying to prevent the mother producing. By giving it at any potential sensitisation events, the mother's immune system rapidly destroys the blood cells without forming its own memory B cells against them. This should prevent future haemolytic disease of the newborn.
Hatshepsut says re Rh Sensitization: Two things: Rh-negative women having abortions are often given a shot of Rhogam. And if the daddy is Rh negative they may or may not give Rhogan. For example, my hub is A- and I am A-, so I skipped the Rh Immunoglobulin. Our baby? A-, FTW! I say "may" because the momma could be lying about who the dad is, I guess. But if both parents are Rh-negative, they can only have Rh-negative offspring.