 It is calculated by internally (not externally) measuring peak uterine pressure amplitude (in mmHg), subtracting the resting tone of the contraction, and adding up the numbers in a 10-minute period.Uterine pressure is generally measured through an intrauterine pressure catheter.Montevideo units can be more simply calculated by summing the individual contraction intensities in a ten-minute period, a process which should arrive at a result identical to the original method of calculation.