YMMV, but I continued to work up until about a week before birth, and my biggest struggle was working around the huge belly. I like to sit close to my desk and had to sit scooted back and turned slightly. It took some adjusting, but eventually I got comfortable. I had to work with this because there was no way to work away from the desk. It helped to have a second monitor, turned toward me at my lopsided angle, lol. The other thing, watch for the feet/ankle swelling! I couldn't even fit in my own shoes because they got so swollen. If you notice swelling, try to keep your feet up, or take frequent breaks to elevate your feet. I kept a tall footstool for this reason. If possible, snag an occasional footrub from your partner, a family member or friend, it really helped me with swelling.
If you're like me and just have to make do with an uncomfortable chair, PILLOWS, PILLOWS, PILLOWS. I ended up puzzle-piecing four pillows onto my chair that really changed things for how uncomfortable it was. Some days I carried my pillow from bed to the desk to make sure I had enough support. Beyond that, take lots of breaks from sitting, drink plenty of water, and remember your snacks. I often had a box of granola bars and a big-ass water bottle right next to my work station.