Thanks to Chris from J-U-B Engineers who sent me this article from Momentum Magazine called Biking With A Belly. Chris had commented on an earlier blog thread about how his wife bicycled all the way through her pregnancy, with her doctor's blessing.
The article says that exercise is essential during pregnancy to help maintain a normal blood pressure, normal blood sugar levels and overall physical fitness, which can help during labor. It suggests bicycling because it is more comfortable for many pregnant women than walking or other forms of exercise. The article also has some tips on how to make cycling safer and more comfortable during pregnancy, such as adapting your bike so you sit more upright and switching from clipless to regular pedals if your balance is off in order to avoid accidents.