Key Point: Studies are all based on pre-1992 machines when the limits were lower.

Fetal Doppler: How to Keep it Safe? (http://www.ncbi.nlm.nih.gov/pubmed/21048451)

Abramowicz JS. Clinical Obstetric Gynecology. 2010 Dec;53(4):842-50. PMID:21048451
Department of Obstetrics and Gynecology and Rush Fetal and Neonatal Medicine Center, Rush University, Chicago, Illinois, USA. Jacques_abramowicz@rush.edu

Abstract

Ultrasound’s record of safety seems to be perfect, with no undisputed reports of adverse effects in humans. However, all epidemiologic studies published so far are based on information obtained with pre-1992 machines, when allowed maximal in-situ intensity for fetal use was increased. Many fetuses are examined very early in pregnancy, a time of greater vulnerability. Doppler can generate much higher level of acoustic energy than B-mode. The thermal index and the mechanical index are indicators of the 2 main potential effects of ultrasound including Doppler. To keep the fetus safe, knowledge of these potential bioeffects is mandatory as is understanding of how instrument controls alter the output. The 2 most important rules are: keep thermal index below 1 and use the lowest possible output for the shortest possible time compatible with obtaining diagnostic information.