title: Reference data representative of normal findings at two-dimensional and three-dimensional gray-scale ultrasound examination of the cervix from 17 to 41 weeks' gestation.
abstract: OBJECTIVE
  To create reference values representative of normal findings on two-dimensional (2D) and three-dimensional (3D) transvaginal ultrasound (TVS) examination of the cervix from 17 to 41 weeks' gestation and to determine the agreement between cervical measurements taken by 2D and 3D TVS.
  
  
  METHODS
  Cross-sectional study covering 17 to 41 weeks in 419 nulliparous and 360 parous women who delivered at term and who underwent 2D and 3D TVS examination of the uterine cervix. We examined approximately 25 women in each gestational week. The length, anteroposterior (AP) diameter and width of the cervix (and of any cervical funnel) and AP diameter of the cervical canal were measured. Results were plotted against gestational age. The agreement between 2D and 3D ultrasound results was expressed as the mean (+/- 2 SDs) difference between the results of the two methods and as the interclass correlation coefficient (inter-CC).
  
  
  RESULTS
  There was excellent agreement between measurements taken by 2D and 3D ultrasound (inter-CC values, 0.80-0.98) but measurements of cervical length taken using 3D ultrasound were greater than measurements taken by 2D ultrasound (mean difference, -0.04 +/- 0.36 cm). Cervical length did not change substantially between 17 and 32 gestational weeks but decreased progressively thereafter. Cervical length was similar in nulliparous and parous women at 17-32 weeks, but from 33 weeks the cervix tended to be longer in parous women. In nulliparae, cervical length decreased from a median of 3.8 (range, 0.7-6.1) cm at 17-32 weeks to 2.3 (range, 0.4-6.0) cm at 33-40 weeks and to 0.7 (range, 0.2-1.5) cm at 41 weeks. In parous women, the corresponding figures were 3.9 (range, 1.0-6.1) cm, 3.0 (range, 0.4-5.7) cm and 0.8 (range, 0.4-3.4) cm (results obtained by 3D ultrasound). Cervical AP diameter and width did not differ between nulliparous and parous women. Median AP diameter increased from 3.0 (range, 2.0-4.6) cm at 17-30 weeks to 3.5 (range, 1.8-5.5) cm at 31-40 weeks and to 4.0 (range, 2.8-5.9) cm at 41 weeks. Cervical width was 3.7 (range, 2.3-6.0) cm at 17-30 weeks and 4.5 (range, 2.3-6.1) cm at 31-41 weeks. The percentage of women with funneling increased from 4% (3/84) at 17-18 weeks to 63% (12/19) at 41 weeks and the percentage of women with an open cervical canal increased from 19% (15/84) to 72% (13/19). Funneling and opening of the cervical canal were equally common in nulliparous and parous women.
  
  
  CONCLUSIONS
  Reference data provide the basis for studies of pathological conditions. Common reference values for nulliparous and parous women can be used for cervical AP diameter and width from 17 to 41 weeks and for cervical length from 17 to 32 weeks. Separate reference values for cervical length for nulliparous and parous women should be used from 33 to 41 weeks.
authors.affiliation: Obstetric, Gynecological and Prenatal Ultrasound Research Unit, Department of Clinical Sciences, Malmo, University of Lund, Sweden.
authors.forename: L
authors.initials: L
authors.lastname: Rovas
authors.forename: P
authors.initials: P
authors.lastname: Sladkevicius
authors.forename: E
authors.initials: E
authors.lastname: Strobel
authors.forename: L
authors.initials: L
authors.lastname: Valentin
mesh_terms: D000328:Adult; D002584:Cervix Uteri / Q000000981:diagnostic imaging*; D003430:Cross-Sectional Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D021621:Imaging, Three-Dimensional; D010298:Parity; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D012016:Reference Values; D018709:Statistics, Nonparametric; D016216:Ultrasonography, Prenatal / Q000379:methods / Q000592:standards*