Originally Posted by J_9
Now, you are right in a sense that the mother's body will "reject" the baby. It's more like the mother's body sees the fetus as an infection if you will. But, again, this doesn't happen until the subsequent pregnancies.
I'm trying to explain it as easily as possible, bear with me, I've been studying measles, mumps, rubella, chickenpox etc all day for unit test tomorrow.
Now, Mom gets pregnant, and we don't know the baby's blood type until it is born. It could be either Rh-negative or Rh-positive. So, let's say this baby is born Rh-positive. Well, Mom's body has built up an antibody to the Rh factor during the first pregnancy. When she gets pregnant the second time the antibodies recognize the Rh-positive as an invader and attempt to fight it off, much like we would with a cold or flu. So, the Rhogam shot is given to stop this process (called phagocytosis).
Again it is VERY common and nothing to worry about at all as long as the doctor knows the factors of both parents.