Placental Migration and Mode of Delivery in Placenta Previa: Transvaginal Sonographic Assessment During the Third Trimester

Abstract

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 postion, 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.

Placental Migration and Its Role in Prediction of Route of Delivery

“…A slower rate of placental migration was directly related to a lesser placental distance from the internal cervical os [8,[13][14][15]. These findings are similar to those of a previous report in which the mean rates of placental migration were 0.3 and 5.4 mm/ wk for those patients who had CD versus normal vaginal delivery, respectively [5,7,[9][10].…”

“…It is thought that this process is not a true migration of placental tissue but, rather, a degeneration of the peripheral placental tissue that receives a suboptimal vascular supply and has slow placental growth in betterperfused uterine areas at the same time, so-called placental trophotropism [4][5][6]. By the nature of the process that involves placental tissue growth, it should be exponential and dependent only on gestational age.…”

“…In at least one quarter of all cases of placenta previa or lowlying placenta suspected on the basis of transabdominal sonograph y in the second trimester, the diagnosis will change after TVU is performed [3,5,7].…”

“…In cases of suspected placenta previa or lowlying placenta, the first step in determining accurate placental position in relation to the cervix is to perf orm a trans-vaginal-ultrasound-scan (TVUS) on patient whose bladder is empty [4,5,6,7]. In at least one quarter of all cases of placenta previa or lowlying placenta suspected on the basis of transabdominal sonograph y in the second trimester, the diagnosis will change after TVU is performed [3,5,7].…”

“…Placenta previa can be defined as one that covers the internal os of the cervix and lowlying placenta as one of which the edge is 1mm to 20mm of the internal os [3]. A distance of more than 20 mm from the lower placental edge and cervical internal os in the second trimester is a normal finding that does not represent a lowlying placenta and does not require followup.In cases of suspected placenta previa or lowlying placenta, the first step in determining accurate placental position in relation to the cervix is to perf orm a trans-vaginal-ultrasound-scan (TVUS) on patient whose bladder is empty [4,5,6,7]. In at least one quarter of all cases of placenta previa or lowlying placenta suspected on the basis of transabdominal sonograph y in the second trimester, the diagnosis will change after TVU is performed [3,5,7].…”