Measuring fetal heartrate using spectral Doppler violates the ALARA (as low as reasonably achievable) principle of imaging, which is the concept that we should try to minimize harm (use low energy) whenever possible. While ultrasound is generally believed to be harmless, Spectral Doppler increases the amount of energy being delivered to the tissue which could be transferred to heat energy. M-mode can obtain the same information at a much lower thermal index.

Appropriate Measurement Using M-Mode

Inappropriate measurement using Spectral Doppler