Ive experienced sciatic pain off and on the last two years from a sports injury. My chiropractor suggested doing squats (even while pregnant) to strengthen and stretch the muscles in that area.
I experienced the same through my first pregnancy, and started feeling it with my second this week (9 weeks). I find it helpful to lay down and cross my effected leg over my other leg (that leg is straightened) and pull the straight leg towards me. Similar to creating a 4 shape with your legs. That stretches those muscles in lower back and buttocks area really nicely. Hold for about 30 sec and repeat. I do this every morning and night. Prenatal massages are very helpful too. Tell therapist what you're experiencing and they can dig in and help release the muscle tension.