Pro-choice advocates sometimes defend their view by saying something like, "We don't know when life begins." The implication is that if we are uncertain about the moral status of the unborn -- whether abortion does, in fact, kill a valuable human being -- then abortion should be permitted.
Pres. Barack Obama, for instance, has said the question of whether the human embryo or fetus has human rights is "above [his] paygrade," yet he strongly believes that killing the embryo or fetus should be legal for any or no reason.
But this doesn't make much sense. If we are unsure whether or not a building to be detonated has a person still inside, it is gravely irresponsible to proceed with the detonation. Similarly, if a woman is unsure about whether abortion kills a valuable person or just a meaningless clump of cells, then she should not have the abortion. It would be terribly irresponsible if she did, a risk of unjustified homicide.
So ignorance works in favor of the pro-life view, not the pro-choice one.
In any case, appeals to scientific ignorance are simply not justified because we aren't ignorant: we know that a new human organism, a member of our species, comes into existence at conception. The question is not when the life of a human being begins (a matter of biological fact), but whether or not human beings at certain stages of development are valuable and deserving of protection, like the rest of us.