Before 20 weeks you can take a pill and usually within a few days the abortion is complete.

After that they have to do a D&C where they dilate the cervix and remove the fetus/placenta. This can also be done before that if a woman doesn't want to have cramping and what not. I don't think the actual procedure changes much between late second trimester and third trimester.

The fetus does get more developed over time. I don't think the third trimester is necessarily a cut and dry stop-point but it's getting close. We can keep fetuses outside of mom alive now sometimes as early as 24 weeks but it often comes with horrible complications for the child. By 34 weeks a fetus has a great chance of getting to childhood without complications if given appropriate medical support. I kind of lean closer to 32-34 weeks because of that. That's 8 months in but remember that technically gestation is 40 weeks long (we count 2 of the weeks where mom isn't actually pregnant yet).

I base my endpoint based on when fetuses can survive outside of mom without serious risk of complications. But what you make that determination based on changes a lot of things.

Is there some specific thing that happens early in the third trimester that really humanizes the fetus to you?