Hi there. No bike riding wouldn't cause birth defects. the only issue is risk of falling/crashing and that could do harm to the pregnancy. I would say that is more down the road when you are bigger and off balance. I rode bikes at the point you are but more leisurely than for serious exercise. good luck
No bike riding would not cause birth defects or bad for pregnancy. I use to take spin class, which is bike riding, 2-3 times a week an hour at a time. Any exercise is good. Just don't over do it if you haven't been riding/exercising and slowly increase over time.
I'm 31 weeks and ride a few times a week about an hour and then rest and another hour back home. Just be very cautious about falling