I agree with those above who have mentioned avoiding lifting weights during pregnancy. The exception would be if you had been lifting weights before you got pregnant, feel free to continue with a similar routine while also paying attention to your body.
To improve the muscles in your back, doing regular life activities is a great idea. Swimming (as mentioned) is a fantastic way to keep the pressure off your joints and stomach while still working the muscles. Also, lifting smaller things and doing light twists was very beneficial to me when I was pregnant.
Best of luck!