Diagnosis of placenta previa during the third trimester: role of transperineal sonography.

Placenta previa can be difficult to diagnose with transabdominal sonography during the third trimester of pregnancy, because of difficulties in imaging the cervix late in pregnancy. Although transperineal sonography offers an additional view of the cervix, its value in the diagnosis of placenta previa has not been studied. Accordingly, we performed transperineal sonography on 164 patients who had had transabdominal scans that had shown placenta previa or had been inconclusive during the third trimester of pregnancy. Transabdominal sonograms had been inconclusive for placenta previa in 157 of these patients because the cervix was not visualized. The remaining seven patients had transabdominal scans that showed placenta previa. Transperineal sonography successfully visualized the internal surface of the cervix in all 164 patients, allowing determination of the presence or absence of placenta previa in all cases. Transperineal sonograms showed absence of placenta previa in 154 patients. At delivery, none of these patients had evidence of placenta previa. Transperineal sonography showed placenta previa in 10 patients. In nine of these patients, placenta previa was confirmed at delivery. The 10th patient did not have clinically significant placenta previa at delivery. Our study shows that transperineal sonography is a valuable technique to complement transabdominal sonography for detection of placenta previa during the third trimester of pregnancy. Use of transperineal sonography should be strongly considered when a definitive diagnosis regarding placenta previa is not possible by transabdominal sonography because the cervix is not visualized. In such cases, transperineal sonography will usually show the internal surface of the cervix without overlying placental tissue, allowing confident exclusion of placenta previa. Occasionally, however, transperineal sonography will show a placenta previa that was not seen with transabdominal sonography.