yea, i think it has to do with the fact that on the machines, i dont think that they can duplicate the way that a mother transfers nutrients to a fetus as efficiently as she could if she were actually alive.
remember, she is brain dead, so the brain is not recognizing that there are babies in her womb. all the machines do are maintain her organs and keep oxygen circulating. her organs can still fail and despite the machines, she can still die.