Starting from the tenth to the sixteenth week of pregnancy ultrasonic placental localizations were performed in 22 pregnant women at four week intervals. In all cases the placenta was located in the anterior wall of the uterus and bordered or covered the internal os of the cervix. In 17 cases a cephalad migration of the placental localization was registered. In 5 cases the low implantation of the placenta did not change until delivery. In 14 of 19 cases marginal atrophic changes of the placenta were found. No morphologic abnormality was detected in five placentas from patients who showed migration of the placental localization. Since the placenta is tightly implanted in the endometrium a true migration of the placenta is unlikely. Our investigations show that areas of low implantation of the placenta are stretched with increasing growth of the uterus and then undergo regressive and pressure atrophy changes. It is likely that the cervical margin of the placenta cannot be detected by ultrasound in a large number of cases.