I thought you could ride them until the 3rd trimester?
Well, technically you can, but you have to take it easy and be careful to not get them too hot. You've also got to be really careful during the first 45 days, while the embryo is still free floating or you can cause them to abort. Then you're limited after the foal is born but before it's weaned. I'd just rather let a brood mare be a brood mare and ride something else if I could. You spend too much time and money to do otherwise.