Interobserver variability of midtrimester fetal nuchal thickness

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 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 > or = 5 mm to a group at risk for Down's syndrome, a high concordance between experienced ultrasonographists was observed (98% agreement; kappa = 0.90).

CONCLUSIONS

Interobserver variability in NT was acceptable between experienced ultrasonographists.

Criteria for fetal nuchal thickness cut-off: a re-evaluation

An attempt has been made to establish a more effective cut-off criterion for nuchal thickness (NT) and to assess the optimal gestational period for the prediction of trisomies 21 and 18. Reference intervals were established for NT from the tenth to the 18th week, using either gestation-specific centiles or the parametric method. The measurements in 47 consecutive trisomy 21 and 18 trisomy 18 cases were plotted against these intervals. Assaying different cut-off criteria for both the centile and the parametric methods, sensitivities and false-positive rates for each gestational week were calculated and then compared with the commonly applied 'two-stepped' cut-off method (3 mm early, 6 mm later). The parametric method, based on a progressive rise, with +2.5 SD for the corresponding gestational week as a cut-off value, showed the best performance (likelihood ratio 38) in the prediction for trisomy 21. The optimal gestational age was the 12-18 week period, with an overall sensitivity of 62 per cent (23/37) for an average false-positive rate of 0.7 per cent. For trisomy 18, the most effective cut-off was also +2.5 SD, and 10-13 gestational weeks as the optimal period, achieving 86 per cent (6/7) sensitivity for a 1.9 per cent false-positive rate.

Early midtrimester fetal nuchal thickness: effectiveness as a marker of Down syndrome

OBJECTIVE

Our purpose was to assess the validity of nuchal thickness in the prediction of Down syndrome in early midtrimester fetuses.

STUDY DESIGN

In 1543 consecutive pregnancies undergoing amniocentesis, primarily for advanced maternal age, nuchal thickness was prospectively measured between 13 and 18 weeks and then correlated with the karyotype obtained from amniotic fluid.

RESULTS

With the cutoff value suggested in the literature (> or = 6 mm), 33.3% (6/18) of the cases of Down syndrome would be detected for a 0.1% (2/1424) false-positive rate, with a positive predictive value adjusted to a prevalence of Down syndrome in the general population of 1:3. To increase the sensitivity of the method, the threshold was lowered to 5 mm, achieving a sensitivity of 77.8% (14/18) for a 2.1% (30/1424) false-positive rate and an adjusted positive value of 1:19.

CONCLUSIONS

In early midtrimester fetuses decreasing the nuchal thickness threshold to 5 mm substantially enhances the detection of Down syndrome with a reasonable false-positive rate.

Transcervical chorionic villus sampling beyond 12 weeks of gestation

The aim of this study was to evaluate the feasibility of transcervical chorionic villus sampling (CVS) after 12 completed weeks of pregnancy. The transcervical route for CVS, using thin biopsy forceps, was accomplished in 335 procedures in the study group, between 12 + 1 and 15 + 0 weeks. Cases were subdivided into two subgroups, the late (13 + 0 to 15 + 0 weeks) and intermediate (12 + 1 and 12 + 6 weeks) subgroups, and the results were compared with a previous series of 1202 early (8 + 0-12 + 0 weeks) transcervical CVS. A successful diagnosis was made in 97.3% of the cases. No differences were found in diagnostic success between the study subgroups, in comparison to the early transcervical CVS series. However, success rates in the study group increased from 90% in the first 60 procedures to 98.9% in the remaining cases (p < 0.001). Amniocentesis was eventually performed in nine cases (2.7%). All cases were rescanned 2 weeks after the procedure, and fetal demise was detected in five cases (1.5%). The spontaneous fetal loss rate up to 28 weeks was 3.1% (2.7% for intermediate and 4.0% for late subgroups). No cases of transverse limb reduction defects were observed. The results of our study suggest that the transcervical route for CVS beyond the 12th week of pregnancy has a satisfactory diagnostic success rate, and an acceptable pregnancy loss rate.