I think you're referring to Anti-D??
As PP said, if this is your first pregnancy then there is minimal risk to the baby if you do not have any internal bleeding during the pregnancy. I am R Neg blood type so have to have the Anti-D injections throughout pregnancy. The main concern is if you bleed during pregnancy and then if the baby has a Pos blood type and it crosses to you, your body can then 'attack' the next baby, which is why you have the anti-D after birth if they are pos. I've never had any problems, both my boys are Pos blood types & I'm on my third pregnancy. If you have your anti-d injections, notify your OB or doctor immediately if you have any bleeding and make sure it's noted on your hospital admission that you are neg blood type then there is nothing to freak out about.
Relax and enjoy the ride