Hmmmm well I had a friend ride down in the NRHA Futurity this October, she is due early February!
It depends on your comfort level really. I've heard that the babies like the motion of riding.....ever wonder why babies wake you up in the middle of the night kicking you in the ribs....it's because you're still and they wake up! But the constant motion of walking or riding seems to lull them to sleep in the womb
If you start feeling unsettled or off balance in the saddle it might be time to take a break until baby arrives