Statement on Measurement of Fetal Heart Rate


Besides watching the monitor and actually counting heartbeats, M-mode is the only ultrasonographic method recommended for measuring the fetal heart rate.* The measurement of the rate in M-mode 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 recorded as a short video clip. Because there is no indication for heart rate to be “heard” by spectral Doppler, spectral Doppler should not be used to measure heart rate.


When attempting to obtain fetal heart rate with a diagnostic ultrasound system, AIUM recommends using either M-mode or a B-mode scan, keeping the thermal index as low as possible, preferably at or below 0.7, and not prolonging the procedure beyond what is necessary to obtain the measurement. Use TIS for the thermal index if pregnancy is less than 10 weeks; use TIB if 10 or more weeks. Fetal heart rate should be documented by M-mode or 2D video clip.  

* Hand-held Doppler instruments without imaging capability are not the topic of this statement.

Approved: 11/05/2011; Reapproved: 10/30/2016

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