Congratulations in advance to both you and your husband, Carolyn. Of course, you will send us pictures of the baby.
Now, I found Nick's answer extremely useful, and I think that he pretty much gave you all the information that you need. I just wanted to point to a specific danger in Aikido that does not exist in most other martial arts: projections. A fellow student once crashed into me one day. I'm not pregnant, but he did slam into my bad knee, and it hurt. I think that there will be moments when it will be not safe for you to be on the mat. Weapon classes should be less dangerous, but I did get wacked more than once by a stray bokken wielded by member of a team training next to me.
Now that you got me into thinking of it, no one is totally safe anywhere, pregnant or not.
Look, just be careful.
And don't forget the pictures.