I've been thinking a lot about riding and pregnancy lately, it is a somewhat controversial topic but I have decided to keep riding until I start getting uncomfortable or unbalanced. Planning to stick to just Joy, no jumping just flat work and walking trail rides soon, although I would love to get miss Joy to one last mini horse trail in the second weekend of October but I am going to wait and see closer to the date. I feel comfortable with my decision to keep riding as long as I can since I am a secure rider and will be riding a horse I am comfortable on.
I am currently12 weeks barely showing (looks like a food baby!). My balance isn't changed and my energy levels are better then early in the first trimester, although not as high as pre-baby. The biggest risk of handling horses while preggo is getting kicked so I am sticking to handling my horses who I know their behavoir quirks and feel comfortable handling.
I think it is a really personal decision and a lot depends on your skill level, awareness of your animals and comfort level.