Treatment with focused ultrasound waves softens the rat cervix during pregnancy

Application of focused ultrasound stimulation (FUS) to the rat cervix during pregnancy has significant physiologic effects. One-millisecond-long pulses of 680-kHz ultrasound with a repetition frequency of 25 Hz, at ISPTA (spatial-peak, temporal-average intensity) of 1, 2 and 4W/cm(2), were applied to the rat abdomen over the cervix. FUS produced a significant change in cervical elasticity known as softening, which is part of the ripening process, comparable to the degree seen just before delivery. Timed-pregnant Sprague-Dawley rats (n = 40) were used. During gestation, the FUS system was applied to the cervix for variable times up to 1 h. Daily measurements of cervix light-induced florescence were made to estimate changes in softening. In addition, cervical stretch estimates of softening were made of isolated cervices of control and FUS-treated rats to measure distensiblity. The ultrasound power with ISPPA (spatial-peak, pulse-average intensity) of 40 W/cm(2) was considered tolerable; the U.S. Food and Drug Administration regulatory limit is 190 W/cm(2) for both the body periphery and the fetus. This is the first report of alterations induced by ultrasound in the connective tissue of the cervix and suggests the therapeutic application of ultrasound for the facilitation of labor and delivery.