I'm A- and need the anti-d injection while pg. Also once Nina was born they had to test her blood type straight away, as she is A+ I had to get another anti-d shot after having her.
The reason for the anti-d shots is that her blood was foreign blood in my system. So next time I'm pregnant, if it's foreign blood again, then my body will attack these and I may lose the baby.

That's a very quick and short way of putting it. Hope it makes some sense.