AIUM Statement on Measurement of Fetal Heart Rate
November 17, 2011
Besides watching the monitor and actually counting heartbeats, there are two sonographic methods of measuring the fetal heart rate: M-mode and spectral Doppler.* The measurement of the rate, both in M-mode and spectral Doppler, is performed by placing a pair of cursors to span a known number of heartbeats on the tracing. At a fetal size (crown-rump length) of 2 mm to 1 cm (approximately 5+ to 7 weeks), the heartbeat can be visualized by B-mode and then "heard" by spectral Doppler ultrasound.
*Handheld Doppler instruments without imaging capability are not the topic of this statement. Statement on Measurement of the Fetal Heart Rate
When attempting to obtain fetal heart rate with a diagnostic ultrasound system, the AIUM recommends using M-mode at first because the time-averaged acoustic intensity delivered to the fetus is lower with M-mode than with spectral Doppler. If this is unsuccessful, spectral Doppler ultrasound may be used with the following guidelines: use spectral Doppler only briefly (eg, 4-5 heart beats), and keep the thermal index (TIS for soft tissues in the first trimester and TIB for bones in second and third trimesters) as low as possible, preferably below 1 in accordance with the ALARA (as low as reasonably achievable) principle.