I find pregnant mares great to ride as for the most part they are very settled and a lot can be accomplished. I think a month is good, but have one mare that lets me know when she doesn't want to be ridden anymore. She just doen't want to go . Each horse is different. There are people who are more active up to due date than others. We give our moms a month or so to bond, once you see the foal starting to stray a bit on it's own, start working slowly as the foal get acclaimated to his mother with someone on her back, and stop to let them nurse. As time goes on that foal will stand and watch. We had one that at a couple months would go be with the other horses, but come time to time to check her out or nurse. Of course one has to be careful to not over do with the mare and use too much energy so she does not have the milk production for her baby. Frequent nursing is important. I think riding a mare with a foal is a good learning process for the foal.