741 women suspected of placenta praevia were submitted to ultrasonic scanning during the second and/or third trimesterr of pregnancy. In 61 an abnormal placental site was diagnosed. During Caesarean 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 week the diagnosis differed in only 4 cases. In all 4 there was a question of a low-lying placenta versus partial placenta praevia. During the second trimester diagnosis of low-lying placenta were revised in several cases at later scans. It is discussed whether these early diagnoses of an abnormal placental site, which later becomes normal, indicate a migrating placenta and/or an uncertainty of the scanning method. Early diagnosis of low-lying placentae should be checked by repeat scans.