I'm not sure about immunisation, but I have done quite a bit of reading about cycling in pregnancy. Not least as I see no reason to stop and my partner goes into a cold sweat at the thought of me being out on a bike. So we have had to examine the information out there together and try and make a decision based on evidence rather than panicked supposition. Essentially, so long as your sense of balance is good (apparently some women can find theirs affected during pregnancy), you're not carrying twins or have some other health reason not to do normal exercise, there's absolutely no reason why you shouldn't. Some sources seem to say don't cycle as you might fall off or be involved in an accident. But only you can assess what you feel the risk to be. After all, you could have an accident driving your car, or be hit by a bus crossing the road. I've been cycling in London for 5 years and the only time I've fallen off was because I was blind drunk! Have a look at this blog about cycling in pregnancy, I found it helpful and inspiring: http://www.guardian.co.uk/environment/green-living-blog/2010/may/21/pregnant-cyclists?showallcomments=true#end-of-comments