I have been struggling to keep my sciatica under control throughout my pregnancy - I have had it before, but it is difficult to keep under control as I can't do all my exercises that normally keep it bearable.
I know people have probably already mentioned these to you, but in case they haven't and in addition to stuff already mentioned on here:
sleep with a pillow between your legs to keep your knees apart
check that you are relaxing your muscles when you are sitting down (esp your bum) - very easy to tense up without realising and relaxing your muscles will help
Keep moving! Go for a walk even if it is painful and someone has to help you... i find that gentle stretching exercises to stretch the back of my legs help, so trying to touch the floor (not easy at 38 weeks) or anything you may have picked up from aerobics etc to stretch your legs can be helpful.
Also make sure you gently stretch you neck and shoulders so that you don't get referred pain.
And keep moving stretching every few hours and even do some stretches when you get up for a wee in the night.
I hope this helps - and if you don't know what stretches to do then i agree it can be good to see a physio,
Hope you feel better soon
x