i rode up until about 2 weeks before i gave birth and the only reason i quit was because i was too big to get on (babe was almost 11 pds)...but, i was young and stupid... i mean brave
....maybe that's why i had such a strong, healthy baby 
nowadays i am a big wimp and i probably wouldn't ride (then again the chance of me getting pregnant is zero!)
my girlfriend rode her horse a couple weeks ago in a parade. the first time for them both. i said aren't you nervous riding your horse in such a crazy atmosphere while pregnant? she said, of course, but, it was wonderful and her horse behaved wonderfully.
i think if you're having a healthy pregnancy, you should keep up your regular exercise. and if that's riding, well, that's what you should do.
--gail
Gailforce -- Another old lady rediscovering her inner cowgirl.