AbstractUltrasonography is the preferred technique for placental localization. It permits a comparatively precise estimate of the separation of the lower placental margin and internal cervical os. Six false positive diagnoses of placenta previa occurred in a series of 244 patients. There were no false negative diagnoses. The diagnosis of placenta previa was confirmed as correct in ten patients. Determination of the location of the lower margin of a posterior placenta obscured by the overlying fetus may be assisted by measurement of the separation of the anterior sacral margin and the fetal head when the latter is dipping into the pelvis. A separation of greater than 15 millimeters usually indicates extension of the lower placental margin over the sacral promontory and onto the lower uterine segment.