Some thoughts:
1) There *are* cases where a difficult decision must be made, for example, cases where conjoined twins are separated, and a decision is made to give priority to one of the twins. That's a rare case, but exists. See, for example:
http://pmj.bmj.com/content/77/911/593.full.
2) But not so extreme are ectopic pregnancies. (There are probably lots of links; here is one chosen mostly at random:
http://www.webmd.com/baby/guide/pregnancy-ectopic-pregnancy.) These types of pregnancies are not so rare, and might rarely lead to a viable pregnancy.
3) The odds of an ectopic pregnancy is about 1/50 (from what I read). Common enough to require an answer as to what is to be done when it occurs.
4) I did a search for "non-viable" pregnancies, since ectopic pregnancies are but one example. Distilling useful information would take a longer time than I have at present, so I have no additional comments.
5) This is arguably still in the realm of medical necessity. Which leads to an initial question, of whether abortion in these types of cases is lawful or moral.
6) Also arguably, other abortions are chosen in cases where there is no medical necessity. That is, when the mother and fetus are both healthy and the pregnancy is expected to reach term. The same question can be asked in this case (if the separation of cases is accepted).
7) Some delineation can be made in elective cases: Eggs and Sperm in-vitro but not combined. In-vitro fertilized eggs. Implanted fertilized eggs, then further delineations based on the transition of fertilized egg to a small bundle of cells, to a fetus, to a newly born fetus.
8) At what point in the transition of uncombined eggs and sperm to birth a transition occurs. I believe that most people would consider the uncombined eggs and sperm to be not a person. I also believe that most people would consider a nearly born fetus is a person.
9) One can try to determine when a person is present. This seems difficult: The transition is gradual. Fertilization and implantation do provide distinguishable steps, and there are key transitions during later development. But, most seem to think that a just fertilized egg is not yet a person, putting the moment of transition in a difficult to place spot during a long and gradual development.
10) This looks at the basic biology. There are other issues which relate to empathy and association. I'll try to think through all of that and add to these notes.
11) There are still other issues which have to do with questions of control and decision making power. Still more to think through.
Thx!
TomB