Placental implantation at 18 weeks and migration throughout pregnancy.

BACKGROUND We wanted to find the most frequent site of placental implantation at 18 weeks' gestation and placental migration during gestation. METHODS Placental location was identified in 2,526 singleton pregnancies at 18 weeks' gestation and characterized into nine groups. Placental migration in 1,336 of these pregnancies was assessed by serial ultrasonography. RESULTS At 18 weeks' gestation, posterior, high placental implantation was more common (45.1%) than anterior, high implantation (42.1%). Relocation of posterior, high placentas farther fundally (16.9%) was three times more likely than farther fundal migration of anterior, high placentas (4%). CONCLUSIONS Posterior fundal placental implantation is more common at 18 weeks' gestation than anterior implantation. Posteriorly implanted placentas are more likely than anterior placentas to migrate farther fundally during gestation. The greater relocation of posterior placentas farther fundally suggests a greater growth of posterior versus anterior uterine wall smooth muscle.