title: Is the third trimester repeat ultrasound scan for placental localisation needed if the placenta is low lying but clear of the os at the mid-trimester morphology scan?
abstract: BACKGROUND
  It is unclear whether women with a low-lying placenta (not overlapping the internal cervical os) at the mid-trimester scan need follow-up. The aim of the study was to determine the rate of placenta praevia, vasa praevia and cord prolapse in the third trimester in this cohort of women.
  
  
  METHODS
  A retrospective cohort study of women with a documented low-lying placenta (<30 mm from the internal cervical os) at the mid-trimester morphology ultrasound scan was done. A composite outcome of rate of placenta praevia, vasa praevia or cord prolapse at term was evaluated. Multivariate analysis was performed to investigate the variables influencing the composite outcome. In addition, a meta-analysis of methodologically similar studies was performed to investigate the effect of the placenta to os distance at the mid-trimester scan on the rate of placenta praevia at term.
  
  
  RESULTS
  One hundred and eighty-one women with a low-lying placenta not overlapping the os at mid-trimester scan were identified. The composite outcome was documented in 20 (11.0%) women, including placenta praevia in 15 (8.3%). Based on multivariate analysis, multiparity, distance from os < 10 mm and antenatal bleeding were independently associated with the composite outcome. Meta-analysis demonstrated significant reduction in rate of placenta praevia for every 10-mm increase in placenta-os distance at mid-trimester.
  
  
  CONCLUSIONS
  Our study supports the recommendation to follow up all women with a placenta lying <20 mm from the cervical os at mid-trimester.
authors.affiliation: Mater Mothers Hospital, South Brisbane, Australia.
authors.forename: Shveta
authors.initials: S
authors.lastname: Kapoor
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Thomas
authors.forename: Scott G
authors.initials: SG
authors.lastname: Petersen
authors.forename: Glenn J
authors.initials: GJ
authors.lastname: Gardener
mesh_terms: D000368:Aged; D002584:Cervix Uteri / Q000000981:diagnostic imaging; D005260:Female; D006801:Humans; D015999:Multivariate Analysis; D010920:Placenta / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D010923:Placenta Previa / Q000000981:diagnostic imaging; D011247:Pregnancy; D011248:Pregnancy Complications; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D012189:Retrospective Studies; D012307:Risk Factors; D016216:Ultrasonography, Prenatal / Q000379:methods
keywords: low-lying placenta; placenta praevia; placental localisation; ultrasonography; vasa praevia