another idea to help with sciatica is to use a tennis ball to rub into the bum area - either sit or lay on it and move around so the ball pushes in to the sore bit. Or you can stand up against a wall with the ball on your bum cheek and push into the wall. it kinda looks like you are trying to scratch your bum on the wall, but it can relieve a bit of the pressure.

I had sciatica bad with the last pregnancy, but it got better after baby was born.