Diagnosis of low-lying placenta: can migration in the third trimester predict outcome?

Objectives To investigate the relationship between the rate of migration of a low-lying placenta during the third trimester and the eventual route of delivery. Methods All patients with a placenta lying within 3 cm of the internal cervical os or overlapping it on transvaginal ultrasound at greater than or equal to 26 weeks' gestation were included in the study. The exact distance between the center of the internal cervical os and the leading edge of the placenta was measured by transvaginal sonography, repeated at approximately 4-week intervals until delivery. Results The mean rates of migration in patients who bad (n=7) and who did not have (n=29) Cesarean section for placenta previa were +0.3 mm/week and +5.4 mm/week, respectively (P<0.0001). When the placental edge was initially > 20 mm from the internal os, migration occurred in all cases and no Cesarean section for placenta previa was performed. For those between -20 mm and +20 mm, sufficient migration to avoid Cesarean section occurred in 88.5% of cases. Beyond a 20 mm overlap, significant placental migration did not occur and all patients required Cesarean section. Conclusion Placental migration may occur progressively, throughout the third trimester. The initial position of the placental edge and the subsequent rate of migration can be used to predict the eventual route of delivery.