Low-back pain is caused by the weight of the growing baby.
What to do to prevent it:
Ask someone to rub or massage your back.
Ask your family for help with some of the heavy work.
Take care to stand and sit with your back straight.
Sleep on your side with a pillow or rolled up cloth between your knees.
Do the 'angry cat' exercise for a few minutes, 2 times each day, and whenever your back hurts: Start on hands and knees with back flat. Push the lower back up. Return to flat back. Repeat.