My Dr. Said as long as your body is used to it already you're pretty safe to continue doing it. Same thing with if you were active in other ways -hiking, running etc. I'm not a Dr but this is just what mine said. I rode with my first baby and was jumping even in the first trimester (probably not smart but we were both okay!) and then had to quit riding somewhere in the 3rd trimester because it was just too uncomfortable.
Due to all my "hard" work
Alli is a horse OBSESSED girl! She's addicted to horses at 3 years old. I'm so proud of her! First word "neigh" - first real sentence "Mommy - we ride patience day?" (her pony is patience) - LOVE it!
That being said I'm trying to get preggers again and am a trainer & instructor. I plan on riding until it's nearly impossible but will stop riding young horses I'm not familiar with after the 1st trimester.... hope this helps!