ELSEVIER European Journal of Obstetrics & Gynecology and Reproductive Biology 72 (1997) 27-29 Interobserver variability of midtrimester fetal nuchal thickness Antoni Borrell a'*, Dolors Costa b, Ruben D. Delgado a, Josep M. Martinez ", Carme Borrell c, Albert Fortuny ~ ~'Prenatal Diagnosis Unit, Department of Obstetrics and Gynecology, Hospital Clinic, University oJ Barcelona Medical School, Villarroel 170, 08036 Barcelona, Catalonia, Spain bGenetics Service, Hospital Clinic, Barcelona, Catalonia, Spain CMunicipal Health Institute, Barcelona, Catalonia, Spain Received 11 July 1996; revised 18 September 1996; accepted 28 October 1996 Abstract Objective: To assess the interobserver variability of ultrasound measured fetal nuchal thickness (NT). Study design: NT was measured in 103 consecutive early midtrimester pregnancies (13-18 weeks) by three ultrasonographists with different levels of expertise, in our Prenatal Diagnosis Unit. Measurements were obtained prior to amniocentesis, blindly in relation to other examiners' values. The Lin's concordance correlation coefficient and the Kappa statistics were used to assess concordance between observed values. Results: Mean differences between pairs of observers ranged from 0.44 to 0.57 mm. A higher concordance correlation between measurements was obtained between the more experienced observers (r=0.77 versus r= 0.51). When assigning cases with a NT .>_5 mm to a group at risk for Down's syndrome, a high concordance between experienced ultrasonographists was observed (98% agreement; K = 0.90). Conclusion: Interobserver variability in NT was acceptable between experienced ultrasonographists. © 1997 Elsevier Science Ireland Ltd. Keywords: Down's syndrome screening; Interobserver variability; Nuchal thickness; Ultrasonography 1. Introduction Fetal nuchal skinfold thickness (NT) measurement has been suggested as a useful ultrasound marker to screen for Down's syndrome in the second trimester of pregnancy [1]. Initial findings pointed out that 43% of the fetuses affected by Down's syndrome were detected, being only 0.1% of the euploid fetuses selected by the same criteria. However, subsequent studies showed a wide range of variability in the results. In prospective studies sensitivity for Down's syndrome ranges from 16 to 75%, for false positive rates from 0.3 to 8.5% [2-7]. In our center, a 78% sensitivity was achieved in a series of 1543 consecutive cases undergoing midtrimester amniocentesis, for a 2.1% false positive rate [8]. * Corresponding author: Tel: +34 3 2275400, fax: + 34 3 2275454. One of the main reasons suggested to explain such differences in the results, was interobserver variability. To our knowledge, few studies have attempted to clar- ify this issue [3,9]. In our Prenatal Diagnosis Unit, 103 consecutive cases were assessed by ultrasonographists with three different levels of expertise in order to estab- lish the interobserver variability of NT. 2. Material and methods Fetuses (n = 103) corresponding to consecutive cases of amniocentesis performed in our Unit during April and May 1994 were studied. Previous to the procedure, NT was measured in each fetus by three observers with different levels of expertise: high (Observer A), interme- diate (either Observer BI or B2), and low (C). Levels of 0301-2115/97/$17.00 © 1997 Elsevier Science Ireland Ltd. All rights reserved PII S0301-21 15(96)02659-0