Bringing a mare back into work after foaling?

Hello! A lot of factors come into this. What was she doing before she was in foal, and how fit was she? The key thing to remember is that to carry a rider again, her abdominal muscles need to rebuild their strength. So I would focus on how she carries herself, and the rider, before starting any trot work. In the early stages, the trot is about working in a good outline so she strengthens the muscles that do the carrying.

Groundwork in hand is excellent, and it also lets you check that she is carrying herself well. Using something like an EquiBand on a mare who has been on maternity leave is a good way to go. It is hard to say exactly when you can add more load, because it depends entirely on the individual horse in front of you. So tune in to how she feels and err on the side of going too slowly rather than too fast.