I am insured with AAMI and this is what their website says.

Does driving while I am pregnant or after an operation affect my policy?
No. If you are concerned, however, that your ability to drive safely may be affected in any way by your condition, we recommend you consult your doctor. We will not pay, however, for loss or damage caused when driving your car after receiving medical advice that your ability to drive is impaired by your condition or medical treatment.

It makes sense to me. Saying can't drive past a pre-defined point in pregnancy would be pretty limiting, almost like saying you can't drive when you have a cold in case you sneeze!