PLACENTAL IMPLANTATION AT 18 WEEKS AND MIGRATION THROUGHOUT PREGNANCY

Background. We wanted to find the most frequent site of placental impl antation at 18 weeks' gestation and placental migration during gestati on. Methods. Placental location was identified in 2,526 singleton preg nancies at 18 weeks' gestation and characterized into nine groups. Pla cental migration in 1,336 of these pregnancies was assessed by serial ultrasonography. Results. At 18 weeks' gestation, posterior, high plac ental implantation was more common (45.1%) than anterior, high implant ation (42.1%). Relocation of posterior, high placentas farther fundall y (16.9%) was three times more likely than farther fundal migration of anterior, high placentas (4%). Conclusions. Posterior fundal placenta l implantation is more common at 18 weeks' gestation than anterior imp lantation. Posteriorly implanted placentas are more likely than anteri or 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.