
I don't know much about blood types in dogs or cats but in humans there's a clash of blood types (I think one involves the mum being - and the baby being +) where their blood types are incompatible and the baby needs a transfusion at birth. Only the second + children are affected, not the first.
Whilst there are many causes of 'fading puppies' one cause in border collies is now known to be TNS (Trapped Neurophil Syndrome) - there is a DNA test for this so worth having done (in Australia and also now Optigen in USA do it). No excuse for the litter born last year with a pup suffering from TNS - a sad disease where the pups usually don't live much beyond 4 months.