light headed, dizzy, nausea and headache were the warning signs he said to look out for. Any of those were keys to stay off of your back.
If you felt fine doing suppine work, however, then it was not an issue to be concerned with.
I was flat on my back doing bench and tricep work (I gave up on crunches for abs after month 6) until 2 days before I delivered. My little guy is a healthy little chunker!