First of all, I didn't propose this 12 week limit (nor do I agree with it, but that's a separate issue). The issue isn't autonomy over their bodies, but the rights of unborn humans.
Lets be specific here. Would you say its OK to kill a live baby right after birth? (IE infanticide)?
OK, if you think that should be illegal, how about abortion of a an eight pound pre-term fetus that is 39 weeks old? (IE one day before its expected to be born). Would you agree that this should be illegal too? If not, please state why not.
OK, how about 37 weeks? 36? 35? 34?
The point is, you have to draw the line somewhere. . .exactly where is subject to another debate, I won't get into in this post.
But whatever your limit is. . .be it 12 weeks or 32 weeks, you make it into law, and enforce it the same way the current 24 week law is enforced in most states.
It becomes illegal to perform an abortion after 12 weeks, and if you do so you're subject to criminal charges. Since doctors generally won't risk their livelihoods and freedom over this, the supply of post-12 week abortion providers dries up and post-12 week abortions become much harder to obtain, and much rarer. What exact charges? Not that it really matters, but practicing surgery without a medical license carries a criminal sanction ranging from 1-5 years in prison in most states. The criminal penalty associated with illegal abortion varies from jurisdiction to jurisdiction, but it definitely can carry jail time.
Do you find this concept difficult to understand too?
Let me add that as a matter of practice most doctors who do perform abortions won't perform them as late as 24 weeks, except under extraordinary circumstances, even when they are legally permitted to do so.