Sonography of the cervix during the third trimester of pregnancy: value of the transperineal approach.

Transabdominal sonography reliably depicts the cervix during the first and second trimesters of pregnancy, but cervical visualization becomes increasingly difficult as the third trimester progresses. To evaluate the possibility of using a perineal approach to bypass the fetus and image the cervix during the third trimester, we used transperineal sonography to image the cervix of 158 third-trimester patients in whom transabdominal visualization was inadequate. The internal os and upper cervix were successfully visualized on transperineal sonograms in all 158 patients, but the region of the external os was obscured by rectal gas in 22 (14%) cases. In some patients in whom the external portion of the cervix was obscured by bowel gas, the problem could be overcome by scanning with the patient in the lateral decubitus position or by rotating the patient. Transperineal sonography is an effective technique for imaging the cervix during the third trimester of pregnancy, allowing cervical visualization in most patients in whom transabdominal sonography of this area is unsuccessful.