Professor Kenji Tanaka at the Juntendo University, Tokyo was one of the very early pioneers in employing ultrasound as a diagnostic tool in the late 1940’s, for the detection of intra-cerebral tumors. This work was with Dr. Toshio Wagai, resident neuro-surgeon in the department and later Professor and President of WFUMB（1976）, AFSUMB（1985）and JSUM（1982）. In obstetrics and gynecology, Professor Ian Donald published, in 1958，the first ultrasound diagnosis of an ovarian tumor, at about the same time as the first works with A-mode in gynecology were presented by Michio Ishihara and Hajime Murooka from Tokyo. Questions regarding safety of this new technology arose from the beginning. Diagnostic ultrasound（DU）has an excellent record of safety in obstetrics（and in general）. Being a form of energy, however, ultrasound has effects in all insonated tissues（bioeffects）, caused by two main mechanisms: thermal and non-thermal（also known as mechanical）. On-screen indicators of these potential bioeffects are the Thermal and Mechanical indices（TI and MI）. Biological effects of DU have been reported in animals but no harmful effects have been demonstrated in human epidemiological studies. Yet, essentially all epidemiological studies so far are based on information obtained with pre-1992 machines, when acoustic output of DU instruments was allowed to be increased, for fetal use, by a factor of almost 8. Many applications are used to scan the fetus and, increasingly, this is performed early in gestation, a time when the fetus is known to be particularly sensitive to external influences. Dating and location of the gestation and verification of the number of fetuses have long been employed in clinical obstetrics. Several more recently described applications include, for instance, early structural anomalies survey and genetic disorders screening. All are, generally, performed with B-mode, but some more recently reported studies use Doppler technology, known to expose the fetus to much higher levels of acoustic energy. Ultrasound is a crucial tool in clinical obstetrics. It needs, however, to be used with precaution, particularly in early pregnancy. There should always be a medical indication and operators should be aware of safety and potential bioeffects. End-users should follow the rule of the shortest time possible, at the lowest possible output, compatible with an adequate diagnosis（ALARA）. As a rule, keeping the TI and MI below 1 will, almost certainly, not cause deleterious effects to the fetus. Education of the operators is vital to maintain the unblemished record of ultrasound in obstetrics and gynecology.