Our objective was to determine the relationship between the placenta and the internal cervical os in the first trimester. This study included ultrasound examinations performed between 9 and 13 weeks' gestation over a 28-month period. The position of the lower edge of the placenta with respect to the internal cervical os was recorded for each transvaginal ultrasound examination. A total of 77 out of 1252 (6.2%) patients had a first-trimester placenta previa; four cases persisted until term. There were no false-negative diagnoses. Our conclusion is that approximately 6.2% of patients will have a placenta previa in the first trimester. Transvaginal sonography can exclude a diagnosis of placenta previa after 9 weeks' gestation. The likelihood that a placenta previa will persist until term increases if the placenta covers the internal cervical os by > or = 1.6 cm.
