Actually, according to statistics
from my first-year college biology
course, it's more like two-thirds.
For every 100 eggs exposed to sperm:
Regardless of the exact numbers, I think the point is that life processes are inefficient, comparatively speaking. Those who believe you should not use birth control or have an abortion because they believe all children are actively conceived through God's will just don't understand how things work.
God designed the system to run by itself. That way, he doesn't have to actively decide who gets a child and when. I know if I were a deity, I'd want as many things automated as possible. Just because you're omniscient doesn't mean you want to keep track of everything consciously.
I just can't imagine that a God who makes all those decisions personally would let things go as they are. He would not give anyone a child they weren't ready to support, nor would he cause so many conceptions that don't pan out. He also would not be giving people so many children when humans are already straining the resources we have.
Well, that's just my opinion. If you don't agree, ignore it.