I went through sciatica when I was pregnant too, so I can sympathize with you Sylvia!
The chiropractor, & getting Jin Shin Jyutsu, (energetic healing work) were what got me through. I believe acupuncture & Shiatsu would also have helped - but the first two did it for me - though ongoing treatments were the key. As you well know - the baby & often the extra fluid throughout the body are all factors, not to mention having the major weight concentrated on the front of the body & that can add up to less than ideal posture too. It's all part of the package deal!
Don't sit in overly deep cushy furniture either - because aside from generally being tough to get out of anyway, with the baby on board, it usually causes more stress on the lower back. So firmer & shallowing seating actually helps! So does keeping a pillow between the legs if you're a side sleeper & you may want to have a pillow between the arms too. Those full length body pillows can be a great solution.
Back to the Chiropractor for a second - just wanted to share that I went for upwarRAB of 2 or 3 times a week in the last few weeks of pregnancy & I'm firmly convinced it helped turn my little one - as she was lying transverse for a very long time, then flipped headdown but facing forward - until I had several very gentle sessions where he worked on the sacral region (center back triangular bony section of the hips). Whether that did it, or she decided to rotate around on her own I can't be 100% sure of - but at least she was in the best position for delivery when the time came!
Get your significant other to work on your lower back & hips for you too, as that may help, as can any good deep - but slow gentle massage. Sometimes when pregnant you may be highly sensitive to how you like to worked on - so be sure you have good communication. FYI -No deep pressure to the tops of the shoulders & especially not around the ankles, though as there are some sensitive trigger points in those areas to avoid during pregnancy - until you go into labor anyway!
Good luck with your pregnancy, resolving the sciatica, & having a safe & easy delivery - as well as a strong healthy baby!