title: Placental implantation at 18 weeks and migration throughout pregnancy.
abstract: 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.
authors.affiliation: Department of Obstetrics and Gynecology, University of Mississippi Medical Center, Jackson 39216-4505, USA.
authors.forename: E F
authors.initials: EF
authors.lastname: Magann
authors.forename: S F
authors.initials: SF
authors.lastname: Evans
authors.forename: J P
authors.initials: JP
authors.lastname: Newnham
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D010920:Placenta / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D011446:Prospective Studies; D012016:Reference Values; D016216:Ultrasonography, Prenatal