It won't hurt you to canter. People had babies travelling West in wagons. As long as you don't fall off, no harm will come to the baby. Trust me. I jogged horses until I couldn't get a leg over the seat of the cart. I stopped the last six weeks and because I was so fit, the labor, first kid, was 45 minutes~!! Exercise is fantastic for you while pregnant. The Doctors knew I was exercising and were all for it.