It's totally normal in the 3rd trimester for those pesky "relaxing" hormones to hit high gear. Your muscles, ligaments, and tendons are Literally softening so that you can birth your baby when the time comes. It's more noticeable in some pregnancies than others (with my last baby I swear it happened from the 1st trimester. My hips kept "sliding" out of joint and it was painful to walk the Whole pregnancy! ugh Thank G-d I'm in better shape this time.

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One thing that you Can do to relieve the backache is to go to the chiropractor. He/she will adjust your lower back & tail bone and get things aligned properly again. It's also important because if your pelvic bones & lower back are out of alignment, the chances of the baby being an odd position are Much greater (which translates to a longer more uncomfortable labor).
The lower back pain has Nothing to do with the possibility of back labor. Back labor is, pure & simple, from the baby being in an odd position during labor. The best thing to do if you Do end up with back labor, is to get on your hands & knees. That position will get the baby off your spine, and encourage him/her to rotate into a better position.