If Mom is Rh
negative and Dad is Rh
positive, the baby statistically has a 50% chance of being Rh
positive. Now, if this is a first pregnancy, it's usually not a problem because the baby's blood and the mother's blood typically do not co-mingle.
During delivery, however, the blood can co-mingle and if that happens, mom will produce anitibodies against Rh, because her body does not have it and recognizes the baby's Rh as foreign. That means she gets "sensitized.