A Sonographic Assessment of Different Patterns of Placenta Previa ???Migration??? in the Third Trimester of Pregnancy

Abstract

Previous sonographic studies have demonstrated a low-lying placenta or placenta previa in the second trimester of pregnancy, which converts to an upper-uterine-segment placenta by the end of pregnancy. How this takes place remains unclear. One possibility is a process of placental trophotropism or dynamic placentation in which the placental margins are atrophied because of a relatively poor vascular supply as other parts of the placenta continue to grow. Another is that the placenta only appears to migrate to a more fundal site as pregnancy advances because the lower urine segment grows more rapidly. In this retrospective study, 163 women with placenta previa, diagnosed by transvaginal sonography at 28 weeks gestation, were followed by serial sonographic studies. The distance from the placenta to the internal cervical os was 0 cm (group 1), 0.1 to 2.9 cm (group 2), or 3.0 cm or more (group 3). The rate of placental migration was reflected by an increasing distance of the lower placental edge from the internal cervical os in millimeters per week. Mean migration rates were estimated at 28 to 32 and 32 to 36 weeks gestation. Migration was considered to have occurred if the placenta accelerated or decelerated late in the third trimester of pregnancy. More than 43% of women were classified in group 3 and were not followed up further. Remaining were 30 women in group 1 and 62 in group 2. Eight women initially placed in group 1 entered group 2 during follow up. At 36 weeks gestation, further placental migration was noted only in group 2 women. Twenty-two women had a final placenta-to-internal cervical os distance exceeding 3 cm, whereas 29 others remained in group 1. No further migration was noted in the women in group 1. The overall mean rate of placental migration was 2.7 mm/week. In general, migration later in gestation followed a pattern of deceleration. Most women delivered at term at a mean gestational age of 38.5 weeks. No vaginal deliveries occurred when the final placental-internal os distance was less than 2 cm. No cesarean deliveries (CDs), either interventional or elective, were done when the final placental difference was 3 cm or greater. Among women with a final placental distance of 2.0 to 2.9 cm, 3 women had elective CD for placental previa without vaginal bleeding, 7 had CD for breech presentation, and 6 had uncomplicated vaginal deliveries. With a decelerative pattern, rates of manual placental extraction and placenta accreta were noted. Interventional CD was carried out in 8 of 10 women whose final placental edge was less than 2 cm from the interval cervical os and who exhibited a decelerative phase of migration. The investigators conclude that a final placental distance of less than 2 cm from the internal cervical os and decelerating placental migration correlate significantly with interventional CD and an increased risk of peripartum complications.