We seem to be getting into an issue which is more a matter of definition than of science. A pupa is not a butterfly – but there will be no butterflies unless there first are pupae.
I believe definitions are also a matter of science. For example, the physicist must provide definitions of quarks and electrons, or else his theories are meaningless. The most precise definitions are found in mathematics, which does not depend on any empirical evidence whatsoever.
Of course, defining a human being is a very difficult thing to do. Socrates defined man as a featherless biped. In response, Diogenes plucked all the feathers off a chicken and declared “Behold, it is a man!” There are many opinions about when an unborn baby becomes a human being, with all of the rights that entails. This is another argument in favor of leaving the issue to the states or local governments, because it is so contentious, and different communities will arrive at different conclusions.
Nevertheless, a comprehensive moral theory should be capable of addressing any ethical dilemma, including very difficult questions about whether it is justifiable to kill an unborn baby. Walter Block’s theory of evictionism attempts to solve the problem of abortion rights by applying the libertarian non-aggression principle. His answer is a sort of compromise, as it would permit abortion up to the point of the baby’s viability outside of the womb, but not after. Therefore, as technological advancements allow babies to become viable at an earlier point in the pregnancy, evictionism will become more restrictive on abortion.