I rode until about 7 months and then felt very uncomfortable. I have known people who carry on right up until the birth! It is very much a matter of choice. If there are health issues during the pregnancy then I would say not to.
Sorry my answer was a cop out, it's just I've not got any bambino's yet, I think if I knew the horse like the back of my hand, I may hack out/school, don't know if I'd jump as my balance is pants at the best of time let alone with a bump!!
I say this now but I think its dependent on many factors, from a human body point of view your body will be producing the hormone relaxin in the first and last trimester, in order for pelvis moving and making space for baby and delivery-so sitting on a horse may be uncomfy.
I rode until around 8monthsish, when it was just too hard to get on and off. I stopped jumping when the pregnancy was confirmed. Totally personal choice, but I carried on as normal - pregnancy isn't an illness
I think it depends on your horse and the relationship you have with them. My boy is your typical chestnut TB (if you wanted to stereotype him) and he does put in the odd buck of excitement/spook etc. However I trust him implicitly and rode until I was 7 months pregnant at which point it hurt after I had got off!!! The strange thing was that he knew I was pregnant and for those 7 months, he was a quiet as a lamb! Personal choice I would say. I stopped jumping and tried to ride with someone else, especially in teh later stages.
I stopped riding as soon as i found out. *i would do the same again also* I couldnt live with myself if something happend to the baby and it was/could have been my fault for riding/falling off. Also I was very uncomfortable even at 6/8 weeks.
The horse wont take any hurt if you dont ride for 1 year, my youngster certanly didnt ;p He's better for the break acually.