Analysis of information sources in references of the Wikipedia article "Obstetric ultrasonography" in English language version.
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.
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.