I would have thought it was a bad idea. They inflate with a great force. Why not ask your doctor/midwife. You may need to show them a video of one inflating so they know what it does, can you do this on your phone?
Do you want to wear it because you are worried about coming off. I would say if you do not feel 100% safe on your horse then now is probably the time to stop riding. Better safe than sorry and a few more weeks/months off really won't make that much difference.
All I can say is that when my hit-air goes off I don't feel any restriction and I actually forgot I had it on until I hit the deck and it didn't hurt. I think you are likely to find that any healthcare professional is more likely to say don't use it and point out the risks of horse riding in general rather than risk being blamed if anything did happen.