title: Influence of sampling site on uterine artery Doppler indices at 11-13⁺⁶ weeks gestation.
abstract: BACKGROUND
  Uterine artery pulsatility index (PI) is a key variable in the first trimester screening for pre-eclampsia. The aims of the study were to examine the effect of sampling the uterine arteries at a site distal to the level of the internal os, and to determine a lower limit of peak systolic velocity (PSV) to establish an auditable standard.
  
  
  METHODS
  PI and PSV measurements were performed at 11-13(+6) weeks' gestation at two sites: at the level of the internal os and 3 cm distal to the internal os. Comparative analyses utilised the Student's paired t-test. A 90% reference interval of transformed PSV measurements at the internal os was generated by polynomial regression.
  
  
  RESULTS
  There was a significant reduction in both the PI (14.9%) and the PSV (17.4%) when measured at the distal site compared to the level of the internal os (both p < 0.001). The best estimated 5th centile for uterine artery PSV at 11-13(+6) weeks was 60.9 cm/s.
  
  
  CONCLUSIONS
  PI measurements performed distal to the internal os are significantly lower and will result in inaccurate pre-eclampsia risk assessment. PSV measurements below 60 cm/s are likely to indicate an incorrect sampling site. Development of auditable measurement standards is important to ensure accuracy of prospective pre-eclampsia screening.
authors.affiliation: Maternal Fetal Medicine Unit, Royal North Shore Hospital, Sydney, N.S.W., Australia.
authors.forename: Gus
authors.initials: G
authors.lastname: Ridding
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Schluter
authors.forename: Jon A
authors.initials: JA
authors.lastname: Hyett
authors.forename: Andrew C
authors.initials: AC
authors.lastname: McLennan
mesh_terms: D000328:Adult; D005260:Female; D005865:Gestational Age; D006801:Humans; D011225:Pre-Eclampsia / Q000175:diagnosis*; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011446:Prospective Studies; D016216:Ultrasonography, Prenatal / Q000379:methods*; D055988:Uterine Artery / Q000000981:diagnostic imaging*; D055815:Young Adult