Deposition of heat as a result of loss in an ultrasonic wave may result in damage to biological tissues. The extensive use of ultrasound for diagnostic purposes during pregnancy necessitates the evaluation of thermal risk to a developing fetus during routine clinical exposures. Because of the small ultrasonic absorption coefficient in soft tissues at low megahertz frequencies, temperature elevations exceeding 1 °C are not expected from clinically employed ultrasound systems, and there is no evidence that such small temperature increases can result in deleterious effects. However, when the propagation path includes bone, which is known to be highly lossy, theoretical calculations and experimental work indicate that local heating might exceed 1 °C for realistic clinical conditions. Thus it is imperative to obtain reasonable estimates of the temperature elevation in and around fetal bone in order to assess risk. Because of a lack of measured data for the thermal and acoustic properties of fetal bone, which depend on gestational age, estimates of the temperature elevation resulting from exposure to ultrasound must be based on crude models. A measured quantity for a heat source resulting from conversion of acoustic to thermal energy in an ultrasound field is suggested. The heat source is developed from theoretical considerations, and can be used in the bioheat transfer equation to obtain better estimates of the temperature increase in fetal bone and the surrounding tissues as a result of exposure to ultrasound.