I wouldn't even if I wasn't pregnant. You're entire body is exposed to that much danger vs a car which could protect you from a low impact hit. One of our policemen died after a low speed crash (15 mph) because a piece of metal slashed his main artery in his leg. He bled out. Kind of a freak accident. I used to ride moped and the moped hydroplaned in a parking lot puddle of rainwater and oil and I broke my hands bracing my fall. Not fun but definitely not safe if you are carrying a baby.
How is this even a relevant question? Really?
I had the same question myself. The weather is so nice. I have seen some people ride 4wheelers when pregnant. This is a question I plan on asking my doctor asap.