Why get an abortion? In the cold practical sense, the child's going to die anyway, so the woman's heart has to be broken (if she didn't want the baby, she'd have aborted it already). Why take the chance that God will consider it murder, or that she'll come to believe that herself? I don't have a problem letting nature take its course.
All good points, and I would have no problem with "letting nature take it's course," either. But I would not impose that course on another, who may decide otherwise -- in this particular circumstance.