Diagnostic ultrasound has an enviable reputation for safety, and the lack of evidence for risk has been one of the key factors which has established it as the pre-eminent imaging method in obstetrics. Ultrasound remains one of very few common diagnostic procedures which may be carried out with little regard to safety. By comparison, all radiographic examinations carry known genetic and somatic risks associated with ionising radiation. Magnetic resonance imaging (MRI) investigations have resulted in several hundred cases of superficial burns, trauma from projectiles, and deaths from malfunctioning pacemakers or displaced metallic inserts. Furthermore, these other modes give rise to biological effects in organs other than those being imaged. For example, a radioisotope lung scan exposes the uterus due to radioactive urine, and scattered x-radiation exposes the ovaries during a chest computed tomography (CT) examination. By comparison, all bio-effects from ultrasound are restricted to the scanned region of tissue.