Ultrasound's record of safety seems to be perfect, with no undisputed reports of adverse effects in humans. However, all epidemiologic studies published so far are based on information obtained with pre-1992 machines, when allowed maximal in-situ intensity for fetal use was increased. Many fetuses are examined very early in pregnancy, a time of greater vulnerability. Doppler can generate much higher level of acoustic energy than B-mode. The thermal index and the mechanical index are indicators of the 2 main potential effects of ultrasound including Doppler. To keep the fetus safe, knowledge of these potential bioeffects is mandatory as is understanding of how instrument controls alter the output. The 2 most important rules are: keep thermal index below 1 and use the lowest possible output for the shortest possible time compatible with obtaining diagnostic information.