Has there ever been a case of a sperm or an egg by itself turning into a human?
No more than cases of a week old fetus by itself turning itself into a person. Neither are people yet, so imo neither should be assigned rights based on what they could become in the future.
If a particular week old fetus would never develop into anything more than a week old fetus, would abortion of that fetus still be immoral or illegal? ETA: as you support pro-choice to 16 weeks, what if a 30-week old fetus stopped developing and would remain in that state the rest of the mother's life? Could it be aborted?
While I agree there is always going to be some ambiguity as to exactly when the combination of an egg and DNA become a human being, I think setting it at birth is no more rational than at a day, a week or a month after conception. Doing so is a pragmatic decision which allows certain members of society to kill human beings they don't want around in the future. We allow this and not the killing of other helpless human beings because there is no chance of us ever becoming a foetus, but we might ourselves become an unconscious human being one day.
Sure, and there are DNRs if we decide we don't want to live in that state; or in some localities families or doctors can decide it's best we die.
The rationality/pragmatism of assigning rights at birth is solely (for my view) because at that point the mother can abandon all care-taking, and the state will take over. If instead the state refused to take over care, I'd support legal infanticide up until the point I think personhood is established. Related to that, if a technology developed that could "transport" (ala a Star Trek transporter) the fetus from the mother to an incubator, and the mother's responsiblities would end, I might support that as the alternative to carrying to term rather than abortion. But that isn't the choice, so I support the killing of the fetus if that's what the mother wants as an alternative to carrying to term.
If it is valid to judge a culture on how it treats its weakest members then I think we are all hypocrites and monsters. I can live with this view of myself. Can you?
I don't consider a fetus a member of society at all. I have far more moral dilemma justifying eating meat or wearing leather than I do justifying abortion, because I think cows have more "personhood" traits (individual personality, self-awareness, conscious appreciation of pain) than fetuses.