I am seriously considering breeding my mare, the foal would be a keeper, and before any decisions are made I will have a conversation with the vet.
I have read other post that people have continued to show until the mare is uncomfortable. But what if anything do you change about your routine. For example, mare is a hunter, she gets bute or banamine when we show, she is on pentosan, I give her msm orally as a supplement. Would you continue your usual protocal or would you stop or change your protocal.
The horse is an only horse, and she is the best she has ever been, and a blast to show, so if I could continue to show a extra few months I would love to, but I am not willing to risk her comfort or the foal, if that is the route we take.
The consensus you will always hear is that if the mare is fit for what you're planning to do with her going in, then competing or working during the first and most of the second trimester is virtually never a problem. Of course you have to factor in the stress of traveling and exposure to lots of other horses, but IMO if the mare is fit, likes her work, and you don't go overboard it's fine.
I competed my TB until almost the end of her 7th month, including hunter trials, Novice eventing, dressage, and even cubbing. I plan to do the same with her daughter, who was no worse for wear, if I'm lucky enough to get her in foal this year, although the schedule will be lighter simply because of lack of time on my part.
FWIW the fetus is about the size of a loaf of bread until about month six--not much for the mare to carry.
I have my mare off all meds right now, just to be extra safe. I don't want to give her even her monthly Pentosan right now due to the short term risk, however small, of bleeding. You'd be better off asking the vet, though, about drugs and supplements.