Someone else had asked this I think in the main forum... I answered with several posts with lots of pics...LOL. Actually, I believe they were asking how soon after giving birth a mare could be worked and how to do so with a foal.
I have often started mares in harness that were pregnant. I've never had issues as long as I was taking it easy. However, I have had mares that were too re-active and I never qot them to the point where they were hitched until later after they had safely foaled. Here's a pic of Bell's first hitch to a wheeled vehicle - she'd been pulling a tire, a pole and had LOTS of ground driving experience in our pastures, in the unfenced pasture and on the roads of 2.5 mile loop. The day we hitched her - was a very laid back day and our daughter who had no driving experience (but is an experienced rider) drove her right off.
It was a "squish" to get her in the shafts! But I kept driving her pretty much daily. I had fixed the one tire on the ez entry cart (shafts slightly wider) and know that on 9 June - I spent about 45 minutes driving her - walk, trot, extended trot and canter work. She appeared to be having no issues and was quite content. She got hosed off after the workout. I fed ponies between 0630 & 0700 - and was going to drive her again... Got tied up in the house until around 9 am and here is what I walked out to...
The larger colt is now much smaller than Bell's filly - he'd been born in February. Don't have any notes as to driving much for a little while - then took the 19 day old filly (GG) with us to driving lessons. She joined us in pair driving lesson.
and here she is on 3 August - making her just short of 2 full months old.
and that "filly" driving with her dam as a 3 yr old...