Eh, at 39 weeks with pain like that reaching your back, please call your doctor or go to the hospital straightaway, okay? Better safe than sorry, they need to check if this is the real thing or just Braxton Hicks, sebab the pain part is the difference. I have a few mum friends who went through similar at the end, and some of them were actually in early labour already! At your stage, any tightening with proper pain especially going to your back should be checked. Don't wait lah, go get it assessed, even if it turns out to be just practice contractions. Your baby is almost here, so let the doctors make sure everything is progressing properly. All the best, mama!