if the mother has a rh- (ie a- b-) then there is the risk that the mothers body will create antibodies against the child. This only becomes a problem if the child's blood enters the mother - such as at birth.
during pregnancy it isn't a problem because the placenta controls the blood passing between mother and child.
this is helped by the administration of an injection (in the UK called anti-d) typically given at 28 and 36 weeks of pregnancy and also again a few hours after birth.
this D-antigen mops up any of the baby's Rh blood cells that have entered the mother and destroy them before she becomes sensitised to them.
In a first pregnancy this is unlikey to cause a problem even if untreated because she has not yet developed sensitivity to the Rh cells.
however in a subsequent pregnancy she may already be sensitised and so her body may see the child as a foreign body.
Simple answer is - pregnancy is not a problem, because there is the anti-D injection to safe guard against it.
So as long as she has proper medical care through her pregnancy then this will not be an issue.