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We found no credible enhancement of neonatal mortality in mice exposed in utero on Day 8 of gestation to ultrasound at 0.44 W/cm2 (+/- 20%) spatial average intensity, 2 MHz, continuous wave, for 60--180 s in a 37 degrees C water bath. This result is compared with the adverse effect on neonatal survival reported by Curto [in Ultrasound in Medicine (Plenum, New York, 1976), Vol. 2, pp. 535--536] for exposure on Day 14 of gestation at about 0.125 to 0.5 W/cm2, 1MHz, continuous wave, for 180 s in a 30 degrees C bath. It is concluded that a temporary reduction in fetal viability, lasting from 4 days to less than 10 days after exposure, could account for both sets of data. It is also evident from these data that improved methods of determining intensities within mammalian tissues are highly desirable.

Pregnant mice were exposed to ultrasound (continuous wave, 2 MHz) on Day 8 of gestation to determine effects on the progeny. The most significant finding was a decrease in mean uterine weight of the female progeny. The thresholds for this effect were 140 s at 0.5 W/cm2 and 60 s at 1 W/cm2, which were below the thresholds previously reported for other effects in mice. We suggest that this indicates a delay or impairment of maturation of the mice exposed in utero. Exposure of the dams to spatial average intensity of 1 W/cm2 for 40 and 60 s had no effect on body weight of the progeny, compared with sham-treated controls. In this experiment the body weights of progeny from sham-treated controls were significantly lower than those from untreated controls on Days 10, 17, and 25 of age. After exposure in utero to 0.5 W/cm2 for 180 s, statistically significant decreases in mean body weights of the neonates were observed, but only on Day 25 of age, in both sexes compared with sham-treated controls. At necropsy at Day 25 of age, neonatal organ weights relative to body weights were not significantly affected for the thymus in either sex or for the seminal vesicles and tests ion comparison with sham-treated controls.

At five stages of gestation, mice were exposed dorsally to ultrasonic irradiation at 1 W/cm2 (spatial and temporal average), 2 MHz, and 34 and 37 degrees C (temperature at beginning of exposure). The results indicated that ultrasonic exposure up to at least 100 s is not hazardous to a pregnant mouse or its offspring in utero under the conditions of our experiment. Thresholds for both undesirable and lethal effects on the dam and its embryos or fetuses appeared between 100 and 200 s. Temperature rises in the uterus to more than 40 degrees C and similarity of results obtained with continuous-wave and burst modes of irradiation suggest a thermal mechanism as one cause of these effects. Preliminary conclusions are that thresholds for both tissue damage to the dam and deleterious effects on pregnancy were similar; however, they occurred at spatial average intensity and exposure levels far in excess of those used in clinical diagnostic medicine.