Bioeffects after exposure to ultrasound are correlated to its duration. Although diagnostic ultrasound has been suggested to induce apoptosis, the underlying signal transduction pathway remains elusive. In this study, women in the first trimester of pregnancy were exposed to transvaginal diagnostic ultrasound with 5.0-MHz frequency for 0, 10, 20, or 30 min. The chorionic villi were obtained 4 h after exposure and activation of caspase-3 and cytochrome c release were analyzed by Western blotting. In contrast with the 0- and 10-min groups, cleavage products of active caspase-3 and cytochrome c release significantly increased in 20- and 30-min groups in a time-dependent manner. We show that long-duration exposure to transvaginal ultrasound activates effector caspase-3-mediated apoptotic cascade of chorionic villi in the first trimester of pregnancy. This occurs through the intrinsic death pathway involved in cytochrome c release. Our findings provide a molecular rationale for discriminant use of transvaginal ultrasound at the early stage of pregnancy.