Generally as a rule of thumb during pregnancy you can do whatever exercise you were doing prior to falling pregnant within reason. So if you had never ridden a bike in your life & were all of a sudden doing 5km then it'd be a problem. Seeing as you're a regular rider it should be ok to a point. Once you start to get bigger though your centre of gravity changes which will make you more susceptible to tipping your bike over & hurting yourself. You might be the best bike rider ever, but your balance is affected in pregnancy. I guess take it the same way as all things in pregnancy, be careful & know your limits.