title: How the cervix shortens: an anatomic study using 3-dimensional transperineal sonography and image registration in singletons and twins.
abstract: OBJECTIVE
  The purpose of this study was to use a fixed reference to study movement (displacement) of the cervical internal os from the second to the third trimester in singletons and twins. The rationale was to gain insight into anatomic changes associated with cervical shortening.
  
  
  METHODS
  For each patient, 2 transperineal scans were performed 12 weeks apart (20 and 32 weeks). The internal os and symphysis pubis were visualized in the same field of view. Image registration techniques were used to align the 2 scans using the symphysis as a fixed reference. Total displacement, anterior displacement, and inferior displacement of the internal os were measured. Displacements were correlated with cervical shortening. Bland-Altman plots and interobserver intraclass correlation coefficients were calculated.
  
  
  RESULTS
  A total of 42 healthy participants were studied: 28 with singletons and 14 with twins. The mean ± SD values for total displacement were 2.1 ± 1.2 and 2.0 ± 1.2 cm for singletons and twins, respectively (P = .75). The direction of displacement was significantly different. The mean anterior displacement was 1.1 cm greater for singletons than for twins (95% confidence interval, 0.29-2.0 cm, P = .01). Mean inferior displacement was 1.3 cm greater for twins than for singletons (95% confidence interval, 2.2-0.1 cm; P = .03). Only inferior displacement correlated with cervical shortening (P < .001; R(2) = 0.74). For every 2.2 cm of inferior displacement, the cervix shortened 1.0 cm. Assessments of reliability showed good agreement between 2 observers.
  
  
  CONCLUSIONS
  The anatomic position of the internal cervical os depends on gestational age and fetal number. Cervical shortening correlated most strongly with inferior displacement.
authors.affiliation: Division of Maternal-Fetal Medicine, Tufts Medical Center, 800 Washington St, 360, Boston, MA 02111, USA.
authors.forename: Reshma
authors.initials: R
authors.lastname: Parikh
authors.forename: Atur
authors.initials: A
authors.lastname: Patel
authors.forename: Trevor
authors.initials: T
authors.lastname: Stack
authors.forename: Simona
authors.initials: S
authors.lastname: Socrate
authors.forename: Michael
authors.initials: M
authors.lastname: House
mesh_terms: D002584:Cervix Uteri / Q000000981:diagnostic imaging*; D016009:Chi-Square Distribution; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D012016:Reference Values; D015203:Reproducibility of Results; D016216:Ultrasonography, Prenatal / Q000379:methods*