Identification of mechanisms responsible for fetal weight reduction in mice following ultrasound exposure.

This study challenges existing interpretations of the effects of ultrasound on fetal development. The results demonstrated that a lower mean fetal weight in mice observed after intrauterine exposure to 2 MHz ultrasound resulted from maternal distress, and was not simply a direct effect of ultrasound interaction with the fetuses. At day 8 of gestation continuous wave ultrasound was applied to different regions of the maternal abdomen at spatial average intensities between 0.5 and 5.5 W cm-2 for 3 min. Insonating the uterus directly resulted in resorptions, abortions and gross developmental abnormalities, with rare instances of fetal weight reduction which were apparently associated with maternal distended bladder syndrome (DBS). When the ultrasound beam deliberately avoided the uterus, by a dorsal approach centred on the vertebral column, mean fetal weight at day 18 was significantly lower in all mothers showing DBS. Fetuses of insonated mothers not affected by DBS were not significantly different from the sham-insonated group whether the uterus was directly exposed or not. The fetal weight reduction endpoint was therefore found to be more closely correlated with maternal damage (DBS) than with ultrasound intensity.