The incidence of placenta previa in mid-pregnancy diagnosed by ultrasonography is abnormally high in comparison with its incidence during labor. The cause of this discrepancy has not been fully elucidated. Detection of the cervix is fundamental to the diagnosis of placenta previa. In the present study, the detection rates for three methods, abdominal, rectal and vaginal scanning were compared and the time course of mid-pregnancy placenta previa was traced. The cause of the above-mentioned discrepancy was thus elucidated. 1. Rate of cervix detection 1) In the 12-23 gestational week period, the cervix was successfully identified in 52.0-62.5% of cases by abdominal scanning, and in 85.7-87.5% by transrectal scanning. 2) The uterine isthmus was not identified by transabdominal scanning. 2. Changes in ultrasonographic findings with the passage of time 1) Transrectal scanning