Just as there are various different blood types, there is also something called an Rh factor. This is a type of protein on the red blood cells. If you have the Rh factor, you are said to be Rh positive. A simple blood test can determine whether you are Rh positive or negative.
If an Rh-negative person’s blood comes in contact with Rh-positive blood, the Rh-negative blood can become sensitized and produce antibodies to the Rh factor as if it were an invading foreign body. Therefore, if an Rh-negative mother becomes pregnant with an Rh-positive fetus, the woman’s body may produce antibodies to attack the baby’s Rh-positive red blood cells. This causes the breakdown of fetal blood cells and anemia and is often referred to as hemolytic disease of the newborn. This can be severe causing brain damage or even death to the baby. Once formed, these antibodies do not disappear so this can cause problems in subsequent pregnancies. Sensitization can also occur during a miscarriage, elective abortion or ectopic pregnancy.