J.-
I am a physical therapist- and here are some ideas.
There is a muscle in the lower back/gluteal region that might be pinching your sciatic nerve. To stretch it, there are several ways depending on how big your belly is. For simplicity sake, lets assume it's the left side you want to stretch.
One way is in sitting. Sit close to the front edge of a chair. Keep your right foot flat on the floor. Put the outside of your left ankle on top of your right knee. Keep your lower back arched. For more stretch, place your left hand on the inside of your left knee and press your knee down towards the floor. For more stretch, keep your lower back arched while bending forward at the hips.
You can do a similar stretch in standing by using a counter that is approximately mid thigh height or maybe a little higher. Lift your left leg up and place it on the counter such that the outside of your leg from the ankle to the knee is in contact with the counter. Keep your lower back arched. For more stretch, keep the lower back arched while leaning forward at the hips.
Another way to stretch; get on all fours. Imagine you have a string hanging from your belly button and place your left knee on that place on the bed. Then, straighten your right leg, making sure it is positioned in the area between your left knee and ankle. Slide your right leg back until you feel a stretch in your left butt/hip.
If you can use a little heat on your lower back/gluteal region before you stretch, it might help you relax. Keep in mind, you might also want to consider using ice on the affected area- ice slows nerve conduction velocity and can help alleviate pain. When I got that pain, I'd have to practically freeze my butt cheek before I could go to bed at night. But use what is comfy for you.
Getting on all fours is a great position, it uses gravity to pull the baby off of the nerves in your back.
If you want to save money, physical therapists can set you up with a TENS unit as well (Transcutaneous Electric Nerve Stimulation)- then you aren't paying for both a therapist and chiropractor. Also, I would recommend that your physical therapist is either certified or has special training in women's health issues and if not, find one who is (I myself am not but one who is works in my office). If your physical therapist has not yet recommended pool therapy- ask about it and try it- I think you would get pain relief from it.
These are just some ideas and I'd be happy to answer questions about the stretches I talked about above.
Good luck and I hope you can get some relief.