I want to know about this too-- it does seem kind of selfish to put the baby at such a high risk, but I don't think I could go 9 months with out having at least a few rides.
A friend of mine said you could do almost anything like riding horses, quads, dirtbikes, and some other things up untill 6 months.
I would definitely have a talk with the doctor.. and if my horse is calm enough-- maybe invest in an endurance saddle, and have the baby daddy pony you around? :)
^im not suggesting you do this.