Rh sensitization happens when a pregnant woman with Rh-negative blood is exposed to blood from her Rh-positive baby, usually during delivery. This isn't a problem in her first pregnancy, but if she gets pregnant again with an Rh-positive baby, antibodies in her 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.