I've just started a prenatal work out that has modified planking in it so that your on your knees. This workout is supposed to be for the 2nd trimester and is supposed to be safe. I thought you were told by your doctor to avoid exercise? I think walking would really just be your safest bet. No matter what I would follow back up with your doctor before you do anything if their last advice was to avoid exercise.

I've done planks throughout my pregnancy (27 weeks now), but I also did them before I was pregnant, so it is really just a continuation of my normal routine. If you have any quesitons about exercises that are safe for you, why don't you check with your Dr/midwife. As an aside, I was doing regular sit-ups into my early second trimester and then transitioned to crunches on bosu-ball. But, if your doctor told you to avoid exercise...then you probably should, at least until you talk with them again.