Mission & Constitution
Basic Science and Instrumentation
General and Abdominal Ultrasound
High Frequency Clinical and Preclinical Imaging
Ultrasound in Global Health
Ultrasound in Medical Education
Non-AIUM Courses & Events
Accreditation & Disclosures
Ultrasound in Med Ed Portal
Ultrasound Awareness (MUAM)
Member Directory (Members Only)
Frequently Asked Questions
Access Accreditation Account
Accredited Practice Resources
Directory of Accredited Practices
Endowment for Education & Research
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.