Drtgirl, at least bike around even into your last trimester if you're comfortable walking around, etc. There are cycling women bloggers who blog about their pregnancy while cycling around town....a ton of evidence of cycling pregnant mothers on the 'Net.

You will be grateful to at least cycle around daily with baby in womb. Nothing wrong with that. And you will be fitter than if you didn't do any exercise. Cycling at your own pace, but not racing, while visibily pregnant is not going to happen often in life, so enjoy it and treasure those few months.