I started at 28 weeks. My OB wants me to have them weekly just as a precaution. He may have said something about twice a week toward the end, but I forget!
My OB isn't starting them until 33 weeks unless there is something in the ultrasounds or exams that doesn't seem right. There's a part of me that wishes she would take a more cautious/conservative approach but I have had a very healthy/uneventful twin pregnancy so she's going with the "let's see how it goes and if we see signs things aren't moving in the right direction, we'll do more."
Re: NST?