Fetal exposure from endovaginal ultrasound examinations in the first trimester

Ultrasonic obstetrical examinations during the first trimester are now often performed endovaginally with higher-frequency (5-7.5 MHz) transducers operating closer to the fetus than for transabdominal examinations. To estimate exposure to the fetus, propagation distances were obtained from a retrospective study of 100 normal first-trimester endovaginal B-mode examinations. No significant dependence of attenuation on gestational age was observed. The range of the attenuation estimates was 1.8-10.4 dB. A mean attenuation of 5.0 dB at 5 MHz for an average depth of 2.8 cm resulted in an attenuation coefficient of .36 dB/cm/MHz. Exposure (ISPTA) to the fetus at each gestational week from three ultrasound units was very similar: worst-case values of the 100 cases ranged from 1.2-1.9 mW/cm2, well within the Food and Drug Administration (FDA) guidelines of 94 mW/cm2 for derated focused transducers. Energy density deposited to the anterior surface of the fetus during a typical examination, assuming that the transducer is kept stationary over one area for the entire period of the examination (which is unlikely), ranged from 143-217 mJoules/cm2, within the American Institute of Ultrasound in Medicine (AIUM) recommendations.