Effects of ultrasound on the prenatal development of the rat. Part 1. 3.2 MHz continuous wave at nine days of gestation.

Individual nine-day rat embryos were exposed for 5 or 15 minutes to graded intensities of well-characterized focused beams of 3.2 MHz continuous wave ultrasound. Fetuses were examined for effects at 20 days of gestation. Analysis of the exposure-response relationships indicates an "apparent threshold" for prenatal mortality at 3 W/cm2 (95 per cent fiducial limits 0.5-6 W/cm2). Fetal size was unaffected at any exposure level and there were only a few malformations, which occured at 10.5 W/cm2 and above.