[Continuous ultrasonic observation on the change of location of placenta in placenta previa].

Placenta previa was studied in 50 cases during 20-28 weeks of gestation by ultrasonic scanning. The change of location of placenta was monitored continuously for these cases. The rates of change of location to normal sites were 90% for low-lying placenta, 65% for marginal implantation, 13% for partial placenta previa and 11% for central placenta previa. It is suggested that partial or central placenta previa observed during mid-trimester should be categorized as a high risk state.