I was learning to ride (and doing high-impact step aerobics) when I got pregnant with my first one - I reckon if he could cling on through my incompetent, bone-jarring attempts at trotting, running would be a breeze!
I advise my runners that, if you're already doing a form of exercise when you fall pregnant, it should be fine to carry on, but I wouldn't take up any new exercise (we have a lot of pregnancies and babies in my group). I think it's a question of listening to your body - if you feel ok to run, then do it. Pregnancies aren't that prone to miscarry if they're healthy; look at the women around the world who do heavy physical work throughout the nine months and deliver healthy babies.
