The development of science in the field of neonatology.
It's possible that science will provide information that shifts the abortion debate. Speaking only for myself, I'll happily change my position if I find that (for example) the fetus can feel pain at 30 days, or that brainwaves are detected much earlier than previously thought.
Ultrasounds of the developing human life in its earliest stages should be convincing enough that it is not just a blob of tissues.
It's not; not to me. I haven't seen any ultrasound evidence which changes my current pro-choice stance (aka. I'm for unfettered abortion during the first trimester, I get pretty nervous about it in the second, and I oppose it in the third except for the most extreme situations).
The evidence is clear that abortion terminates a human life.
Well, sorta. The evidence is clear that it terminates life, but whether that constitutes "a human life" is very much debatable. For example, skin cells are a form of human life, and no one has any moral problems with terminating them. A human zygote is also a form of human life. So is an 8 month old fetus. We know that abortion kills something, but whether it's a human being or the tree equivalent of an acorn is hotly contested. And rightly so.
Abortion goes against the natural principle that for a species to grow and flourish, you can't eat your own, so to speak.
You realize there are species that do exactly that, right? Cats and dogs sometimes do it. "Natural principle" is nebulous at best.