I don't know RAGBRAI, but having trained all through a pregnancy and seen many friends do or attempt to do the same I can only say that it's impossible to tell how you are going to feel. You may be one of the lucky ones who feel fit and energetic, you may feel fine in the middle of the day but fall asleep by 6 pm, conversely you may feel like **** in the morning but be raring to go later. Your legs may feel stupendous, but your lungs may be all cramped and squashed up because of the baby appropriating space inside. There's no telling, and some of the fittest people I know have been knocked out by mid-pregnancy, while others have just sailed merrily through. Personally I was very comfortable on a bike right up to delivery, but I wasn't riding long distances at the time.
Best of luck to you, no matter how it goes!
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett