Note that the speed of sound is not a constant. It depends on temperature, humidity and more factors.
See - Speed of sound - Wikipedia
at 25C the SOS = 346
at 0C the SOS = 331
So measuring length with an ultrasound should at least be compensated for temperature.
That is a difference of 5% in speed so also 5% in measured length if a constant would be used.
As the length is squared in the formula for BMI this error effectively doubles.
length = 170 cm (25C) and mass = 75Kg
==> BMI = 75 /(1.7*1.7) = 25.95
length 170 cm would be measured as 170 + 1700.05 ~ 178.5 (0C)
==> BMI = 75 /(1.7851.785) = 23.54
difference in BMI = 2.4 ~ 10% error,
That said, a difference of 25C is a lot and used as an example, however temperature in the morning (minimum) and afternoon (maximum) can differ 5 - 15C quite easily. How big the error will be in your case is left as an exercise.