I realise it doesn't completely co-mingle (it would certainly cause a LOT of complications if it did), but it does enough that I would argue that there was already a transfusion from mother to child and vice versa, even if only an extremely minor one. Otherwise the Rh(D)-factor complications would not happen (the babies blood needs to come into contact with the mother's blood in order for the immune system response to occur), although this is much more likely to be due to a placental bleed rather than an actual co-mingling.

Still, the fact that a minor transfusion can occur without the mother even knowing about it... Well I'll leave the philisophical discussion for another thread.

The only thing I've never figured out is why the Rh(D) factor can cause such complications, but not blood type (A,B,AB,O) (during pregnancy).