The ultrasound that lets doctors look at organs inside the body is different than therapeutic ultrasound. Therapeutic ultrasound uses much lower frequencies, and it could harm a developing fetus. No one really knows what effects therapeutic ultrasound has on a fetus. Some doctors think that the fetus is protected from ultrasound waves by the mother's pelvic bones for about four months into pregnancy. But most doctors would rather be cautious.
There are many other treatments you can try to ease your low back pain. Ask your doctor or therapist about massage, posture training, positioning, or support belts.