title: Uterine artery impedance at very early clinical pregnancy.
abstract: OBJECTIVE
  The aim of the study was to construct gestational age-based reference ranges for the uterine artery (UtA) mean pulsatility (PI) and resistance (RI) indices from 6 to 10 weeks of pregnancy.
  
  
  METHODS
  A prospective, cross-sectional, observational study was carried out in 312 singleton pregnancies with gestational age ranging from 6 to 10 weeks. UtAs were examined transvaginally by color and pulsed Doppler imaging, and the mean of the right and left values of PI and RI, as well as the presence or absence of a bilateral protodiastolic notch, was recorded. UtA-PI and UtA-RI reference percentiles were derived through time-conditional quantile regression.
  
  
  RESULTS
  The authors derived the 10th, 50th, and 90th reference percentile curves and correspondent 95% confidence intervals, for the evolution of the UtA mean PI and RI from week 6 to week 10 of gestation. The prevalence of bilateral notching absence was 8.1% (6/74) at 6 weeks and 28.8% (15/52) at 10 weeks.
  
  
  CONCLUSIONS
  The authors present evidence of progressive reduction of uterine vascular impedance in a very early stage of pregnancy and provide new, averaged UtA-PI and UtA-RI charts between 6 and 10 weeks of gestation.
authors.affiliation: Department of Experimental Biology, Faculty of Medicine, University of Porto, 4200-319, Porto, Portugal; Departamento da Mulher e da Medicina Reprodutiva, Centro Hospitalar do Porto EPE, Largo Prof. Abel Salazar, 4099-001, Porto, Portugal.
authors.forename: Luís
authors.initials: L
authors.lastname: Guedes-Martins
authors.forename: Joaquim
authors.initials: J
authors.lastname: Saraiva
authors.forename: Rita
authors.initials: R
authors.lastname: Gaio
authors.forename: Filipe
authors.initials: F
authors.lastname: Macedo
authors.forename: Henrique
authors.initials: H
authors.lastname: Almeida
mesh_terms: D000293:Adolescent; D000328:Adult; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D011247:Pregnancy; D011261:Pregnancy Trimester, First / Q000502:physiology*; D011446:Prospective Studies; D018617:Ultrasonography, Doppler, Pulsed; D055988:Uterine Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D055815:Young Adult