In reply to JemG:
I used a body harness while pregnant and I don't think this would be a good way to go to help with back pain. It was not very comfortable and the discomfort point was actually accross my back about mid way up especially when I was being lowered, lowering a heavier partner,at a hanging belay or rappelling. I would have hated to fall on it (ok I wasn't leading while I was pregnant, but it was uncomfortable enough when I weighted it to make me think I would not have wanted that to happen anyway...).
A well fitted sit harness should not really aggravate your back.
And a comment about your sciatica. I suffered from back pain for years including, but not limited, to sciatica pain. For many years doctors just told me to do more stretches, sent me for physical therapy and told me that I wasn't flexible enough, had bad posture etc. After the birth of my second child I started to have pain in the achillies and then all over my feet, to the extent that for a couple of years there were times I could hardly walk and I was on maximum dose of ibuprofen most days for the two years. Eventually I was diagnosed with ankylosing spondylitis, which is a form of arthritis,and although not curable can be treated very well with specialized medicines. It took a further year to get the right drugs, but I have been almost pain free for a year and a half now, and after 10 years or so of pain it's amazing to be able to enjoy my children, work and yes, climb again. So if your back pain does not resolve continue to,push your doctors to consider all things that could be contributing, I'm not saying you have what I have, but make sure you fight for yourself to try and get better and actively seek second opinions if you don't feel you are getting what you need fro your doctor.