I rode my good horse a few times early on... He totes 3 yr old kids around and I trust him fully. I did not ride my others at all, since they were younger and I didn't want to take the risk. I wasn't sure how long I would ride going into it, but once I started showing the teeniest bit (I think 4 months?) I stopped. I just looked down one day, saw the little baby bump with the saddle horn right there, and thought NO way is it worth it. Next baby, I may not ride at all. I just don't think it's worth the risk.
Edited to also say..... I think it's definitely a personal thing, and whatever you feel comfortable with. I've seen hugely pregnant women barrel racing. I think that is insane and would never do something so risky... but that's me. I am
a little over protective and paranoid