Rh sensitization happens if you have Rh-negative blood and are exposed to Rh-positive blood. Because the blood types don't match, you can develop antibodies against the Rh-positive blood. If you are pregnant, you can become sensitized if your baby has Rh-positive blood. Exposure to your baby's blood happens most often during delivery. This often isn't a problem in the first pregnancy. But if you get pregnant again with an Rh-positive baby, antibodies in your blood can attack the baby's blood cells and cause serious problems.
A blood test is the only way to know you have Rh sensitization or are at risk for it.