I mean, kids can have a different blood type than their mother. Pregnant women don't show two different blood types when tested for that during pregnancy, do they?

As Sven said, the placenta is a good filter. As I understand it, it works a bit like a heat exchanger: Two different circulation systems in close contact, but kept separate. Oxygen and nutritients are filtered out of one and transferred into the other, and vice versa for waste products (including carbon... uh, dioxide?). On the whole, a remarkable piece of natural engineering for a temporary lump of gunk.

Yeah, I know it's not perfect: Junkie mothers can give birth to already-addicted babies. That shit hasn't been around long enough for evolution to fix it. Diseases have.