Women (741) suspected of placenta previa were submitted to ultrasonic scanning during the 2nd and/or 3rd trimesters of pregnancy. In 61 an abnormal placental site was diagnosed. During cesarean section, carried out in 46 of these cases, the location was compared with the ultrasound findings. The scanning diagnosis during the last trimester correlated with the findings at delivery. At scanning after the 35th wk the diagnosis differed in only 4 cases. In all 4 there was a question of a low-lying placenta vs. partial placenta previa. During the 2nd trimester diagnoses of low-lying placenta were revised in several cases at later scans. Whether these early diagnoses of an abnormal placental site, which later became normal, indicated a migrating placenta and/or an uncertainty of the scanning method was discussed. Early diagnosis of low-lying placentae should be checked by repeat scans.