Ultrasound imaging of the early fetus: is it safe?

Author(s): Jacques S Abramowicz

Diagnostic ultrasound has an excellent safety record in general and, in particular, in obstetrics. As a form of energy, ultrasound has effects in insonated tissues that are caused by two main mechanisms: thermal and nonthermal (also known as mechanical). Onscreen indicators of these potential bioeffects are the thermal and mechanical indices. Biological effects of ultrasound have been reported in animals but no harmful effects have been demonstrated in human epidemiological studies. However, all epidemiological studies published so far are based on information obtained with pre-1992 machines, when acoustic intensities for fetal use of ultrasound instruments were allowed to be increased by a factor of almost eight. Many applications are used to scan the fetus and, increasingly, this is performed early in gestation, a time when the fetus is known to be particularly sensitive to external influences. While dating and location of the gestation and verification of the number of fetuses have long been employed in clinical obstetrics, several more recently described applications include, for instance, early anatomy survey for structural anomalies and screening for genetic disorders. All are, generally, performed only with B-mode, but some more recently reported studies use Doppler technology, known to potentially expose the fetus to much higher levels of acoustic energy, such as analysis of the ductus venosus or cardiac function. Ultrasound is a crucial tool in clinical obstetrics, it allows assessment of fetal anatomy, behavior and function. However, it needs to be used with precaution, particularly in early pregnancy. There should always be a medical indication and personnel should be aware of safety and potential bioeffects. End-users should follow the rule of the shortest time possible, at the lowest possible output, compatible with an adequate diagnosis