The decision about whether to abort should ultimately be the host's. There may be exceptions, but that should be the default. Until it is born, the embryo/fetus is part of the woman's body, and she therefore should have the right to decide what to do with it.
Euthanasia should be legal, although there needs to be some kind of procedure to prevent either abuse (e.g. trying to cash in on a rich relative) or needless suicide (e.g. someone believing a situation is hopeless when in fact there is a solution).
As far as life being sacred... I think if anything should be held "sacred", that would be the spirit or soul (which I use to mean something like "the personality characteristics that define who someone can be"), not the sheer "life" (metabolism) of the spirit's container (body).
The following principles only apply to the extent that the "spirit" exists without a brain:
- Every spirit deserves a vessel (body) worthy of it:
- A spirit deserves a body which is physically able to support it, without unusual suffering (e.g. chronic pain or excessive maintenance).
- A spirit deserves a body capable of executing its wishes.
- A new spirit deserves to be born into a community that wants it and has the ability to care for it adequately until it is able to care for itself.
- Every spirit has the right to choose to end itself.
- If the spirit is too new to be able to make or express its life decisions, then the parents (or the community) must make those decisions.
This is basically an emotional argument, but it's at least as reasonable as the emotional arguments advanced by the "pro-life" movement. For all the moral high ground they claim, they're elevating the body to a position of sacredness and totally ignoring that which ought to be far more important, i.e. the person inside.
A more succinct phrasing might be: forcing a fetus to be born isn't always doing them a favor. Life isn't always the right choice.
In less emotional terms: we need to balance our compassionate wish to preserve life (or, more generally, to maximize happiness) against our compassionate wish not to cause suffering. Those twin considerations should override any arbitrary sacredness of human life at any age. Any life decision depends heavily on context, including the wishes of the life in question (at which we can only make an educated guess in some cases), and should not be decided arbitrarily based on any hard-and-fast rule.
(This leaves open a lot of issues which need to be addressed, such as when do parents abandon their right of choice, when is a parental decision out of bounds, etc. but at least gives some basis from which those decisions can be made.)