By now, your baby weighs 3 3/4 pounds and is about 16.7 inches long, taking up a lot of space in your uterus. You're gaining about a pound a week and roughly half of that goes right to your baby. In fact, he'll gain a third to half of his birth weight during the next 7 weeks as he fattens up for survival outside the womb. He now has toenails, fingernails, and real hair. His skin is becoming soft and smooth as he plumps up in preparation for birth.
Assuming its not preterm labor that's ailing you, you can probably blame your growing uterus and hormonal changes for your aching back. Your expanding uterus shifts your center of gravity and stretches out and weakens your abdominal muscles, changing your posture and putting a strain on your back. Hormonal changes in pregnancy loosen your joints and the ligaments that attach your pelvic bones to your spine. This can make you feel less stable and cause pain when you walk, stand, sit for long periods, roll over in bed, get out of a low chair or the tub, bend, or lift things.