The aim of this study was to investigate the effects of diagnostic ultrasound on chorionic villi during early pregnancy in rats through the examination of heat shock protein 70 (HSP70) expression and apoptosis in chorionic villi. Using TUNEL, RT-PCR, western blot analysis and immunohistochemistry, the extent of apoptosis and the gene/protein expression of HSP70, Bcl-2 and Bax was determined in the samples of the chorionic villus from the rats during early pregnancy. Compared with the unexposed group, there were clear signs that apoptosis had occurred in the villi exposed to ultrasound for 30 min. The expression of HSP70 and Bax in the villi gradually increased with the extended duration of exposure to ultrasound, whereas the expression of Bcl-2 gradually decreased. Bcl-2 expression did not differ significantly between the group exposed for 10 min and the group exposed for 20 min. Taken together, our data demonstrate that HSP70 may protect chorionic villi exposed to ultrasound by inhibiting apoptosis; however, its protective effects are limited over the extended duration of exposure to ultrasound. Thus, the prudent use of ultrasound during the early stages of pregnancy should be advocated.