- As a form of energy, diagnostic ultrasound (DUS) has the potential to have effects on Living tissues, e.g. bioeffects. The two most likely mechanisms for bioeffects are heating and cavitation. Hyperthermia is considered teratogenic in human fetuses during the first trimester Actual temperature changes cannot be studied in the human fetus. The thermal index [TI) expresses the potential for rise in temperature at the ultrasound's focal point. The mechanical index (MI) indicates the potential for the ultrasound to induce inertial cavitation in tissues. Nevertheless, cavitation has not been documented in mammalian fetuses, since there is not an air-water interface, which is needed for the cavitation mechanism. Since an output of TI over 1.5 is a known hazard, the question is: What are the settings in which such hazardous exposure occurs? Our conclusions regarding safety of DUS, based on the data that has been available till now, are the following: (1) Ultrasound end-users are poorly informed regarding safety issues during pregnancy. Further efforts in the realm of education and training are needed in order to improve knowledge of end-users about the acoustic output of the machines and safety issues. (2) First trimester ultrasound is associated with negligible rise in the thermal index. (3) Increased acoustic output Levels, as expressed by TI levels, are reached while performing obstetrical Doppler studies. In particular, TI Levels may reach 1.5 and above. Doppler procedures should be performed with caution and should be as brief as possible during obstetrical ultrasound. (4) Acoustic exposure levels during 3D/4D ultrasound examination, as expressed by TI are comparable to the two-dimensional B-mode ultrasound.