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 cerv ical os or overlapping it on transvaginal ultrasound at greater than or equ al to 26 weeks' gestation were included in the study. The exact distance be tween the center of the internal cervical os and the leading edge of the pl acenta 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 n ot 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 Cesar ean section for placenta previa was performed. For those between -20 mm and +20 mm, sufficient migration to avoid Cesarean section occurred in 88.5% o f cases. Beyond a 20 mm overlap, significant placental migration did not oc cur and all patients required Cesarean section. Conclusion Placental migration may occur progressively, throughout the thir d trimester. The initial position of the placental edge and the subsequent rate of migration can be used to predict the eventual route of delivery.