Fertilization: the process of union of two gametes whereby the somatic chromosome number is restored and the development of a new individual is initiated.
This is basic reproductive biology. The assertion that life begins at
fertilization is a tautology. Might as well be objecting to
a legislative finding that A is A or 2+2=4.
So why find the declaration so discomfiting? Because abortion would
be easier to justify if one were ignorant of basic reproductive biology. If one
imagines gestation as a process in which an inanimate lump of matter, or a part
of the mother's body, or a parasite (which by definition is an organism of a
different species) grows for months before spontaneously generating an
independent identity, then it is hard to formulate a moral objection to abortion
prior to the moment at which spontaneous generation occurs.
Abortion is morally fraught because we know that's not how it works. The
moment human sperm meets human egg, a new human organism comes into existence.
To acknowledge that basic truth is not to resolve the abortion debate, merely to
assure that it is based on scientific fact. The rest of the debate is about
values and practical questions: At what developmental point between zygote and
adult does a human being acquire rights, especially the right not to be killed?
If that point is before birth, how does one weigh the mother's competing rights
claims? (Or the father's, for that matter, though our current legal regime
grants him none even if he is married to the mother.) Apart from morality, what
are the real-world consequences of regulating abortion in particular ways, or of
not doing so?
If your goal is to maximize sexual freedom, then it's expedient to answer the
abortion question in the most permissive way possible. But when your position depends on denying a tautology, you may find it
a difficult one to defend.