Clinical significance of placenta previa detected at early routine transvaginal scan

Transvaginal ultrasonography in early pregnancy was used to determine the t he prevalence of placenta previa and the rate of persistence until delivery . The location of the placenta was registered systematically in 2342 pregna nt women who underwent transvaginal ultrasonography at 10 to 16 weeks' gest ation as a primary examination. The outcome of pregnancy as well as the pre sence or absence of placenta previa at delivery was noted in a total of 215 8 cases. A receiver operating characteristic curve was generated for the di fferent measurements from the edge of the placenta to the internal cervical os versus placenta previa at delivery. In 105 of the 2158 women screened i n the early stages of pregnancy (4.9%) the placenta extended to or over the internal cervical os, and in 34 of 2158 patients (1.6%) the distance to th e placental edge beyond the internal cervical os was equal to or greater th an 14 mm. Of the eight cases of placenta previa at delivery, six (75%) were identified in our study, and two cases were missed. When a cutoff value of greater than 14 mm is used for the receiver operating characteristic curve the likelihood is 17.6% (95% confidence interval = 6.8 to 34.5) for placen ta previa at delivery. Although a high percentage of false-positive results occur owing to the low prevalence at delivery this screening procedure can identify high-risk patients who should be rescanned later in pregnancy..