attenuation factor probably underestimates the acoustic exposure in fluid-filled tissue.A homogeneous tissue model with attenuation coefficient of 0.3 decibels/cm megahertz throughout the beam path is commonly used when estimating exposure levels. The model is conservative in that it overestimates the in situ acoustic exposure when the path between the transducer and site of interest is composed entirely of soft tissue. When the path contains significant amounts of fluid, as in many firstand second-trimester pregnancies scanned transabdominally, this mode may underestimate the in situ acoustic exposure. The amount of underestimate depends on each specific situation.