Excerpt from the National Institutes of Health Consensus Development Conference Statement - 1984
Lack of risk has been assumed because no adverse effects have been demonstrated clearly in humans. However, other evidence dictates that a hypothetical risk must be presumed with ultrasound. Like-wise, the efficacy of many uses of ultrasound in improving the management and outcome of pregnancy also has been assumed rather than demonstrated, especially its value as a routine screening procedure.

Ultrasound Safety references
part of a comprehensive guide to Obstetric ultrasound (Joseph Woo)

ACE Graphics - Ultrasound: More Harm Than Good?

Quality assurance of medical ultrasound
DEGUM-WORKSHOP April 2000 - Quality assurance of medical ultrasound