It's medically possible all the way but whether it's legal or not depends on where you live. In the US it's legal up to week 20 or 24 in the different states. So if it's not legal at 20 weeks in your state you can go to a different state.
http://en.wikipedia.org/wiki/Abortion and also take contact with a doctor who can explain everything in detail and which method that would be best for you.
It's allowed up the to the 24th week for health reasons only. Maybe you should consider adoption this late. Having a abortion this late is like going through labor. You can contact Planned Parenthood...