My mom is an Xray tech at a hospital and a nurse told her that one way to help sciatic nerve pain is pushing hard on your lower bak. Really low though like right at the bottom of your tail bone almost at your butt. It hurts really bad while your doing it but it does seem to help. I usually have my husband push while I'm sitting down or I will lean up against the counter or table. Maybe the bathroom sink? Just push against it as hard as you can on your back side.
My nurse said get on your bed on all fours with a pillow under your head/shoulders, put your face/shoulders into the pillow and raise your bum into the air. This will encourage baby to move slightly higher and take pressure off of that area, which builds up from standing/sitting for long periods of time.
Sleeping with a pillow between my legs and a heating pad for 15 min on low before bed have helped me. If itbgets bad I'll alternate with an iced gel pack. Good Luck.
Also turn the chair around and sit that way, stratle the chair and hang your belly a bit to bring the baby off the nerves. If you have a birthing ball or yoga ball lean on it on your knees, let gravity pull the baby forward.
Re: sciatica pain