Sit on the edge of a chair with your feet flat on the ground a comfortable distance apart. With your back straight and your abs pulled in, extend your arms out to your side at shoulder height, your palms facing backward. Pulse your arms behind you slowly and gently, as if you were trying to squeeze your shoulder blades together. Repeat.
Ensuring proper posture is a key way to eliminate and prevent back pain. Due to the changes the body undergoes, it is important to stretch the hip flexors which get tight from the anterior pelvic tilt as your belly gets bigger. Simply working on pelvic tilts in various positions is a great exercise. Hamstring stretches can decrease sciatic symptoms. These can be done by lying down as well as standing and sitting for the various trimesters.
Doing abdominal strengthening will also help with back pain because your abdominals help support your back and your growing belly. Stretching the front muscles in your chest and keeping your upper back muscle strong will help with upper back pain by decreasing the effects of the postural changes occurring. The "angry cat" exercise is a great one to stretch the upper and lower back and strengthen the abs. This is done on your hands and knees. Exhale and round your back up, pulling in your abs toward your spine, feeling a stretch in your spine. Hold this for five seconds, inhale and return to a neutral spine position.