An apparatus for measuring the volume of fluid in a human or animal body cavity using a non-invasive, ultrasound echo technique, comprising: one or more transducers configured to: transmit at least a first ultrasound signal (A-E) at a first power level into the body such that the at least one signal reaches a portion of the target body cavity and does not generate harmonic components in an echo signal from the at least first ultrasound signal, the at least first ultrasound signal having a first frequency, and transmit at least a second ultrasound signal having the first frequency and at a second power level higher than the first power level; means for receiving at least a first ultrasound echo signal from the body cavity for the at least first ultrasound signal, and receiving at least a second ultrasound echo signal from the body cavity for the at least second ultrasound signal; means for using the received at least first ultrasound echo signal as a reference signal, wherein the reference signal is used as part of a normalization process to eliminate effects of patient variations; means for determining a measure of higher harmonic components of the received at least second ultrasound echo signal, wherein the harmonic components correspond to at least a second harmonic having at least two times the first frequency; means for re-scaling the reference signal based on an increase in power level between the first power level and the second power level; and means for determining a volume of fluid in the body cavity from the measured harmonic components and the re-scaled reference signal by: subtracting the re-scaled reference signal from the at least second ultrasound echo signal to generate a difference signal, filtering non-linear content from the difference signal to generate a value associated with the volume of fluid, and determining, based on the value, the volume of fluid using a look-up table. one or more transducers configured to: transmit at least a first ultrasound signal (A-E) at a first power level into the body such that the at least one signal reaches a portion of the target body cavity and does not generate harmonic components in an echo signal from the at least first ultrasound signal, the at least first ultrasound signal having a first frequency, and transmit at least a second ultrasound signal having the first frequency and at a second power level higher than the first power level; transmit at least a first ultrasound signal (A-E) at a first power level into the body such that the at least one signal reaches a portion of the target body cavity and does not generate harmonic components in an echo signal from the at least first ultrasound signal, the at least first ultrasound signal having a first frequency, and transmit at least a second ultrasound signal having the first frequency and at a second power level higher than the first power level; means for receiving at least a first ultrasound echo signal from the body cavity for the at least first ultrasound signal, and receiving at least a second ultrasound echo signal from the body cavity for the at least second ultrasound signal; means for using the received at least first ultrasound echo signal as a reference signal, wherein the reference signal is used as part of a normalization process to eliminate effects of patient variations; means for determining a measure of higher harmonic components of the received at least second ultrasound echo signal, wherein the harmonic components correspond to at least a second harmonic having at least two times the first frequency; means for re-scaling the reference signal based on an increase in power level between the first power level and the second power level; and means for determining a volume of fluid in the body cavity from the measured harmonic components and the re-scaled reference signal by: subtracting the re-scaled reference signal from the at least second ultrasound echo signal to generate a difference signal, filtering non-linear content from the difference signal to generate a value associated with the volume of fluid, and determining, based on the value, the volume of fluid using a look-up table.