Doctors are not always right when they make this diagnosis, but even when they are, there is an enormous moral distinction between the natural death of a child and the intentional killing of one.  It is the same as the distinction between a man dying from a heart attack or being killed in a holdup. 

The question is, once we have adopted this “going-to-die-anyway” standard, why apply it only to the unborn?  If a man is charged with murder, shouldn’t we drop the charges if we discover that the victim already had a fatal disease?  Or when certain medical experiments are too dangerous to be attempted under normal circumstances, why shouldn’t we force prisoners on death row to participate since they’re going to die anyway?