Three-dimensional transvaginal sonography in third-trimester evaluation of placenta previa

In cases of placenta previa, the distance from the placental edge to the internal os is determinant in deciding on the mode of delivery that will minimize the risk of hemorrhage. The reproducibility and interobserver reliability of this measure are unknown. The internal os is not a point, as two-dimensional (2D) ultrasonography might suggest, but an oval patch that has a measurable width, which may be as wide as the distance separating it from the placenta. It is therefore difficult to determine the exact location of the os using a 2D sectional plane. We report the case of a nulliparous woman admitted for vaginal bleeding at 37 weeks' gestation in whom transvaginal ultrasound examination showed placenta previa, but for which two sonographers reported different measurements for the distance between the placental edge and the internal os. We describe a technique to measure this distance using multiplanar three-dimensional (3D) ultrasound imaging, which allows the smallest distance between the middle of the os and the edge of the placenta, which appears as a line, to be determined. In this case, measurement using 3D ultrasound showed a shorter distance than was suggested by 2D examination. This led us to schedule a Cesarean section delivery, during which there was minimal bleeding. Standardization of 3D measurements should make it possible to define more relevant cut-offs for determining the management of cases of placenta previa.