Some of the best arguments against abortion don't need to take the form of words.
Human, approximately six weeks gestation.
Human, approximately ten weeks gestation.
Human, approximately twelve weeks gestation.
Human, approximately sixteen weeks gestation.
Human, approximately twenty-one weeks gestation.
Human, 23 weeks gestation. You can read her story here.
A great many women have abortions between six and twelve weeks of gestation; to get an even better look at what these unborn humans look like, visit the image and video gallery of this site. I hasten to say, to my pro-abortion readers, that the EHD site is neutral on abortion and takes no position either way--their interest is scientific and technical, not ideological.
In the very earliest stages of pregnancy, before a mom even knows she's pregnant (let alone schedules an abortion), the baby's humanity may not yet be as visible as these images reveal. But that doesn't matter--we are not human because of what we look like, but because of our intrinsic reality. Every one of us was once the same age and size as these little ones, above, and we were as human, and as deserving of the right to live, then as we are now.
UPDATE: Spoke to my sister today--one of my little nephews saw her looking at this post, and said, "Oh, look at all the cute little babies!" Children see clearly what we struggle to recognize.
UPDATE TWO: I have removed images and linked to them instead, for copyright reasons.