Please start reading the general pain page.
Back pain is a very generally felt condition and is not necessary. Generally the use of moxa and on the back and leaning forwards, after drinking water (not chilled, and pure) using a lot of topical magnesium and being well nourished will be easy to work through.
You do not have to wait til the baby is born for relief. In fact your back pain is your body warning you that attention is urgently needed – especially get a lot more magnesium (stop the calcium supplementation for a start) and vastly more Vit D on board – get out in that sun!!!
Perhaps start here and organise yourself a faja – as this will make a huge difference – low on the hips and quite firm.
Next, find a moxa stick and get someone to use this – as demonstrated here.
Perhaps look to the Pregnancy App for more helpful advice.