Placenta previa occurs when the placenta implants in the lower uterine segment and partially or completely covers the internal cervical os. It is diagnosed on ultrasound, most definitively with transvaginal imaging, which provides accurate measurement of the placental edge distance from the os. Complete previa (placenta covering the os) is a contraindication to vaginal delivery. Sonographers must identify the placental position and measure the distance from the placental edge to the internal os as part of every standard obstetric survey.