Two Rh+ parents can have an Rh- baby. It depends what their own ancestry is.
Everyone has two genes for the Rh factor, one from each parent. Rh+ people can have two positive genes (++) OR a positive and a negative (+-). Because the + is dominant, even if they are +-, they will always test positive.
Rh- people ALWAYS have two --genes, because the negative is recessive.
Here is an example.
My dad is + and everyone in his family is +. My mum is -. So all my sibs and I are +- and our blood tests positive.
My dh is exactly the same: his dad is + and his mum is -, so he is also +-. So, even though we are both Rh+, we each have an Rh- recessive factor. Because we EACH have it, there is a 25% chance that any of our babies would be Rh-. There is a 75% chance of an Rh+ baby (25% chance of ++, 50% chance of +-).
For Renee84 above, her DP must be B+- (one of HIS parents or ancestors was Rh-). For them, they have 50% chance of a B- baby, and a 50% of a B+ (actually B+-) baby.

IF THE MUM & DAD ARE BOTH NEGATIVE, THEN MUM HAS NO PROBLEMS AT ALL!! Because in that case, bub HAS to be Rh- too.

Clear as mud eh?