Determination of in situ ultrasound exposures in experimental animals and patients is necessary to assess any potential hazard of ultrasound implied by reported biological effects in animals. A layered model was used to calculate total attenuation of ultrasound pulses as they passed through tissue layers to an organ site, thus enabling calculation of in situ intensities from free-field transducer emissions. The model assumes plane homogeneous layers of tissue of known thickness, attenuation and acoustic impedance. Calculations based on the model, compared with in situ measurements in a human cadaver specimen, were accurate to within approximately +/- 3 dB. We then calculated in situ clinical exposures using published obstetric B-scans. Total attenuation to the fetus averaged approximately 11 dB at 3.5 MHz.