The role of transvaginal sonography in the diagnosis of placenta praevia.

Transvaginal sonography was performed in 70 patients diagnosed to have placenta praevia by transabdominal sonography. The diagnosis was confirmed either by digital examination in theatre at term or operative finding at delivery. Forty-nine cases (70%) were correctly diagnosed to have placenta praevia by both modes of sonography. Transvaginal sonography ruled out placenta praevia in 12 cases (17%) thought to be placenta praevia by transabdominal ultrasound. Both transabdominal and transvaginal sonography demonstrated 'placental migration' in 4 cases (6%) which were no longer praevia at delivery. Five patients (7%) were erroneously believed to have placenta praevia by both sonographic techniques. Overall, the diagnostic accuracy of transvaginal sonography was 92.8% compared with 75.7% for transabdominal sonography. None of the subjects experienced any exacerbation of bleeding or other complications. The results suggest that transvaginal sonographic localization of the placenta is safe and superior to the transabdominal route.