This would only truly be so in the case of ectopic pregnancy, and the best treatment of ectopic pregnancy is the removal of the diseased fallopian tube, not the direct murder of the child. For other indications discussed such as cancer or eclampsia, the mother can generally live long enough to allow her child to have life. So it doesn't seem clear to me at all, even accepting the moral balance of take the child's life or let both die, that allowing the child a few weeks of additional development at risk to the mothers health is enough to justify taking its life.
Better to give both a chance at life than at least one a certain death sentence.
Would you prefer that both die, rather than just one?
I said it before, but I have to keep repeating myself to you. If such a case arises (and again, I am not aware of any real justifications for direct abortion given the current state of medical technology), better two natural deaths than one murder. Why is that so hard to understand for you?