title: Narrow subpubic arch angle is associated with higher risk of persistent occiput posterior position at delivery.
abstract: OBJECTIVE
  To determine whether the subpubic arch angle (SPA) measured by three-dimensional ultrasound is associated with the fetal occiput position at delivery and the mode of delivery.
  
  
  METHODS
  Nulliparous women with an uncomplicated singleton pregnancy at ≥ 37 weeks' gestation were recruited from two tertiary centers between September 2013 and August 2015. All women underwent a three-dimensional transperineal ultrasound examination and the SPA was measured using the previously validated Oblique View Extended Imaging software. Data on the outcome of labor were obtained prospectively in all cases and the correlations between SPA and the fetal occiput position at delivery and the incidence of operative delivery were investigated.
  
  
  RESULTS
  Overall, 368 women were included in the study. Fetal position at delivery was occiput anterior in 339 (92.1%) cases and occiput posterior (OP) in 29 (7.9%) cases. A significantly narrower SPA was found in the OP group compared with the occiput anterior group (104.4 ± 16.8° vs 116.4 ± 11.9°; P < 0.0001). The SPA was significantly narrower in women requiring obstetric intervention compared with in women with a spontaneous vaginal delivery. From multivariable logistic regression analysis, SPA and maternal height appeared to be significant predictors of both the fetal occiput position at delivery and the risk of operative delivery. The best cut-off value of SPA for predicting an OP position at delivery was 90.5°.
  
  
  CONCLUSIONS
  A narrow SPA is associated with a higher risk of persistent OP position at delivery and of operative delivery. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.
authors.affiliation: Department of Obstetrics, University of Parma, Parma, Italy. tullioghi@yahoo.com.
authors.forename: T
authors.initials: T
authors.lastname: Ghi
authors.affiliation: Department of Obstetrics and Gynecology, Sant'Orsola Malpighi University Hospital, University of Bologna, Bologna, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Youssef
authors.affiliation: Department of Obstetrics and Gynecology, Tor Vergata University, Rome, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Martelli
authors.affiliation: Department of Obstetrics and Gynecology, Sant'Orsola Malpighi University Hospital, University of Bologna, Bologna, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Bellussi
authors.affiliation: Department of Obstetrics and Gynecology, Tor Vergata University, Rome, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Aiello
authors.affiliation: Department of Obstetrics and Gynecology, Sant'Orsola Malpighi University Hospital, University of Bologna, Bologna, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Pilu
authors.affiliation: Department of Obstetrics and Gynecology, Sant'Orsola Malpighi University Hospital, University of Bologna, Bologna, Italy.
authors.forename: N
authors.initials: N
authors.lastname: Rizzo
authors.affiliation: Department of Obstetrics, University of Parma, Parma, Italy.
authors.forename: T
authors.initials: T
authors.lastname: Frusca
authors.affiliation: Department of Obstetrics and Gynecology, Tor Vergata University, Rome, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Arduini
authors.affiliation: Department of Obstetrics and Gynecology, Tor Vergata University, Rome, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Rizzo
mesh_terms: D000328:Adult; D036861:Delivery, Obstetric / Q000379:methods*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D007746:Labor Presentation; D010388:Pelvis / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D011247:Pregnancy; D011446:Prospective Studies; D016216:Ultrasonography, Prenatal / Q000379:methods*
keywords: dystocia; fetal occiput position; labor; malposition; subpubic arch angle; three-dimensional ultrasound; ultrasound in labor