With my physio head on my main suggestion would be not to adopt any one static position as eventually you will have pain no matter how you sit. I suggest that you alternate your position every 30mins or so even if this means standing up walking to make a cup of tea then going back and sitting down. I would alternate your sitting between birthing ball, sofa, lying down, an upright chair with cushions/pillows to make it soft. Also a rolled towel/cushion in your lower back will help to give added support.
The best chair for me (seriously) is one of those £2.50 plastic patio chairs from Wilko or similar. You can put a throw over it to hide it if it offends your sensibilities! Roll up a hand towel and place it behind your back for optimum support, then sit down. Ideally you should be a collection of right angles - back at right angle to thighs (which should be parallel to the floor), thighs at right angles to lower legs. Unless you're an amazon it's likely that you'll need to put something under your feet to raise them - I use a Next Directory, it's perfect for me! It's not exactly the kind of thing to stick your feet up in and snuggle up to dh but it's perfect for sitting pain-free.
I'm afraid that baby positioning will have to come second to you and your comfort. 15 weeks is a long time to be in pain and you risk doing some longer term damage if you don't deal with it now.
it's not spd as I have no pain in front, it's all lower back. My chiropractor said my back seemed "very fragile" and that there may be a problem with one of the discs. I feel like i'm in pain almost all the time but was totally unimpressed with my first physio session so haven't been back [feeling a bit guilty about that]. Only thing physio did point out that's been very helpful is that I need to use my transverse abdominals more and I find that doing about 20 pelvic tilts before I sit down and when I get up seem to alleviate pain as it "wakes up" the TA muscles and helps them to hold painful joints together. So I move more easily. Am doing them all the time now as I regularly have very sharp pain.
I did try propping myself up with pillows but I think it's the angle really that causes pain, and also the sofa cushions are quite soft. I've noted point about moving around regularly. I did spend the last few min of "one good turn" doing pelvic tilts last night!
Will dust off bouncy ball and blow up while dd is plugged into Cbeebies.
Ah, I have scoliosis too so maybe we have the same pain? When you get pain does it make you cry out? I guess discussing it won't really help but I do feel a bit down at times because I am waddling/limping like I'm 9 months gone! Also my mother keeps trying to offer me helpful times - when I screamed as I got off the sofa the other day she asked me what shoes I was wearing and later said I needed to try to walk properly