Placenta previa: the role of ultrasound in assessment during third trimester

To assess the role of ultrasound in detecting the migration of placenta previa during the third trimester at Lady Willingdon Hospital and Jinnah Hospital, Lahore during the period July 2000 to September 2002.

Eighty pregnant women with the diagnosis of placenta previa at 28 to 32 weeks of gestation were included in the study. After base line ultrasound, scan was repeated every two weeks until delivery or placental migration for more than 3 cm from internal cervical os. Detailed information for placental position, distance from cervical os and relation to presenting part was recorded. Women with major degree placenta previa were admitted in the hospital at 32 -34 weeks of gestation. Delivery plan was made according to degree of placenta previa by completed 37 weeks of gestation. Cesarean section was done for the women with major degree placenta previa and minor degree placenta previa with antepartum hemorrhage and obstetric indication

Out of 80 women placental migration to a distance of more than 3-5 cm from the internal cervical os occurred in 20 cases (12 anterior/anterolateral, 8 posterior/posterolateral) by 36 weeks of gestation and 20 had complete placenta previa. Out of remaining 40 cases, 12 patients had vaginal delivery and 28 had cesarean section. Placental migration was not observed in women with total placenta previa or posterior placenta previa when the distance of lower edge of placenta was less than 1 cm from the internal os.

Ultrasound is important for the diagnosis of placental localization and placental migration during third trimester. Placental migration takes place more often in anterior than in complete or posterior placenta previa.