Late term abortion for defects (no debate)?

I am wondering how late is too late for late term abortion. Does the law vary by state? I am facing the possibility of having a defected baby and I do not want, nor would I want to carry a baby with a defect like down syndrome. It is my personal choice, and I don't want to be flamed. I am merely wondering what the laws are and how late the procedure can be performed. I am waiting for my level II and amnio appointment. I am 22 weeks.