Ultrasound scanning machines are designed and built to conform to strict international regulations. In the hands of well-trained, competent operators this equipment has proved an invaluable resource, especially since, in contrast to imaging techniques based on ionising radiation, there is no evidence that repeated exposure has cumulative and/or detrimental effects.
Ultrasound has become central in the management of the problems of early pregnancy, particularly but not exclusively in the management of women after assisted conception and those in whom miscarriage is feared or ectopic pregnancy is suspected.
Much effort has been devoted to the issue of the safety of diagnostic ultrasound devices, with particular attention having been paid to the unavoidable heating of tissue resulting from the output of acoustic power. In day-to-day use, operators are trained to limit ultrasound output using the displayed ‘safety indices’. This ensures that the temperature increase in tissues caused by ultrasound equipment should be below 1.5°C. This is a widely adopted safety threshold that is below what is found to be teratogenic over long periods in laboratory studies in mammals.
Ultrasound also has mechanical effects independent of its ability to heat tissue and can produce acoustic cavitation and generate biologically active free radicals. For example, ultrasound may impart momentum to tissues and cause amniotic fluid to flow in the direction of its beam: a phenomenon termed ‘acoustic streaming’. Cells that are bound together will tend to resist this force and thus, on exposure to ultrasound, experience some displacement from their position of equilibrium and lateral forces can produce shear within tissue. While the force is estimated to be very small, and very unlikely to cause harm, the effect on a developing conceptus remains uncertain.
This paper addresses the issues of ultrasound in the embryonic period in the light of the most current evidence and guidance from national and international ultrasound safety committees and organisations. In particular, it will consider both medical and nonmedical use of the commonly used ultrasound modes in the embryonic stage of human development (up to 10 weeks of gestation).