hi,
I think level and intensity of exercise depends on what you have been doing pre-pregnancy, I have had some very fit girlfriends run half marathons when they are 4 months preggers! I think in general it's safest not to do any form of exercise or activity more intense than what you have been doing and maybe taking it down a level. Generally low impact cardio is good for you and the foetus eg. walking on the treadmill or the cross trainer. You should also remember not to over heat and keep heart rate below a certain level (I think it's 140 bpm). Would also highly recommend prenatal yoga (I did it twice a week from week 16 right up to my due date), the exercises helped immensely with backaches, stiffness, water retention etc. Some of my friends hit the swimming pool and they swear by it - it's probably the only time you'll be weightless! Generally, exercise common sense and listen to your body....