I have had to stop driving my truck at work only as the steering wheel sits in a different position then a car steering wheel, I have to move the seat back to get my belly in and then i cannot reach the pedals safetly, as for a car though as long as your comfy and can fit keep going. I drove up to a day before my DS was born and he was 9 days over.
The only rule i know of is dont drive yourself to hospital in labour.