Tissues of the central nervous system are sensitive to damage by physical agents, such as heat and ultrasound. Exposure to pulsed spectral Doppler ultrasound can significantly heat biologic tissue because of the relatively high intensities used and the need to hold the beam stationary during examinations. This has significant implications for sensitive neural tissue such as that exposed during spectral Doppler flow studies of fetal cerebral vessels. Recent changes in the FDA regulation allow delivery of almost eight times higher intensity into the fetal brain by ultrasound devices that incorporate an approved real-time output display in their design. In this situation, ultrasound users are expected to assess the risk/benefit ratio based on their interpretation of equipment output displays (including the thermal index, TI) and an understanding of the significance of biologic effects. To assist in the assessment of potential thermally mediated bioeffects, a number of conclusions can be drawn from the published scientific literature: the amount of ultrasound-induced intracranial heating increases with gestational age and the development of fetal bone; pulsed spectral Doppler ultrasound can produce biologically significant heating in the fetal brain; the rate of heating near bone is rapid, with approximately 75% of the maximum heating occurring within 30 s; blood flow has minimal cooling effect on ultrasound-induced heating of the brain when insonated with narrow focused clinical beams; the threshold for irreversible damage in the developing embryo and fetal brain is exceeded when a temperature increase of 4 degrees C is maintained for 5 min; an ultrasound exposure that produces a temperature increase of up to 1.5 degrees C in 120 s does not elicit measurable electrophysiologic responses in fetal brain; for some exposure conditions, the thermal index (TI), as used in the FDA-approved output display standard, underestimates the extent of ultrasound-induced intracranial temperature increase.