Just my opinion, but, horses in the wild don’t stop work when they have a baby and still do the 25 miles a day. Comparing it to a human being pregnant is not the same.
Last foal, 2 1/2 years ago, I got on my mare three weeks after foaling and slowly started riding her. She was showing 3rd 4 months post baby–baby just stayed home with babysitter and had been used to it for awhile. But, she is a very fit horse and used to doing a lot of work and likes it, or she turns into a raging bitch. She was actually gaining so much weight after foaling I cut back on some grain. She is a very known factor with five babies–it’s a whole different situation with a horse you don’t have a history of breeding for. You need to leave a LOT of wiggle room to see what’s best for her and the foal.
I think you do what the mare tells you, but saying absolutes like you can or can’t do anything is not legitimate. I am breeding again next year and the timing works well so she’ll be 6 month pregnant at championships at 4th/PSG. She’s SO much better pregnant, and has such a big barrel she doesn’t really even show until 8 months. But, I know her and we’ve done this twice before.