Placental migration and mode of delivery in placenta previa: transvaginal sonographic assessment during the third trimester.

Background: The objective of this study was to assess the role of serial transvaginal sonography (TVS) in predicting placental migration and mode of delivery in pregnancy complicated by placenta previa during the third trimester.

Patients And Methods: In this prospective observational study, all the cases had confirmed diagnosis of placenta previa before 32 weeks' gestation. TVS was performed between 28 and 32 weeks' gestation for 287 patients with either clinical suspicion or previous sonographic diagnosis of placenta previa. The lower placental edge was found to cover the internal cervical os, or was found to be within 3 cm from it in 63 patients. A two-weekly TVS was performed for every patient until delivery, or until migration of the lower placental edge to a distance of more than 3 cm from the internal cervical os was observed. Detailed information on the placental position, its distance from the internal cervical os, and its relation to the presenting part were recorded at each examination.

Results: Placental migration to a distance of more than 3 cm from the internal cervical os occurred in 24 patients (38%) by 36 weeks' gestation. Of the 63 patients, 19 (30.2%) delivered vaginally. The last scan of these patients revealed that the distance between the internal cervical os and the lower placental margin were more than 2 cm and 3 cm in the anterior and posterior placenta previa, respectively, and the presenting parts were below the placental margin. Placental migration was not observed sonographically in any of the eight patients with posterior placenta previa when its lower edge was initially located within 1 cm from the internal os. It was also not observed in either the 16 patients with total placenta previa, or in any of the other patients beyond 36 weeks' gestation.

Conclusion: Posterior placenta previa lying within 1 cm from the internal cervical os and total placenta previa do not migrate during the third trimester. On the other hand, other types of placenta previa may migrate but not beyond 36 weeks' gestation. The mode of delivery does not depend only on the placental degree but also on the placental position (anterior or posterior), and the relationship between the presenting part and the lower placental edge.