Typically babies in the last month will settle in head first. Three percent of babies are breech at term. No one knows exactly why babies turn, but in general, the shape of the uterus and the pelvis at term most easily accommodates the baby's head in the mother's pelvis. Babies with a nice amount of amniotic fluid have a great swimming pool to do summersaults and flips. My wife is due to have an ultrasound of the baby next week.
Routine ultrasound uses sound waves that are at a very safe level for imaging of a fetus.
These are at a much lower energy than those sound waves used to break up kidney stones or heat sore muscles. I would have no reservations in having a diagnostic ultrasound performed during pregnancy. Ultrasound machines used for imaging are incapable of producing sound waves powerful enough to do any harm. Constant pelvic and lower back pain is typically from the constant strain on your joints, tendons and bones. Braxton-Hicks are reported as contractions which are intermittent usually up to three to four times per hour.