Impact of nuchal cord on measurement of fetal nuchal translucency thickness

Objective: To define the impact of nuchal cord on the measurement of fetal nuchal translucency thickness (NT).

Methods: Between December 2004 and June 2006, we examined prospectively 53 fetuses that were observed on routine first-trimester ultrasound examination between 11 + 3 and 13 + 6 weeks of gestation to have nuchal cord causing an indentation in the skin in the nuchal region. The fetuses were re-examined after a median interval of 132 min, when the cord was no longer around the neck and indentation of the skin had resolved. Various NT measurements (highest, lowest, mean) with the cord located around the neck (nuchal cord) were compared with NT measurements in the absence of nuchal cord ('true' NT) in the same fetuses. Measurements were considered to be equal when they were within mean +/- 1.96 SD, defined by our own intraobserver repeatability according to the method of Bland and Altman.

Results: The mean of the largest and the smallest of six measurements in the presence of nuchal cord fell within the mean +/- 1.96 SD of our own intraobserver repeatability, i.e. gave a correct estimate of the true NT, in 80% of fetuses, while the NT was overestimated in 10% and underestimated in 10% of fetuses. The largest and the smallest of six measurements in the presence of nuchal cord gave an underestimate of the true NT in 2% of fetuses and an overestimate in 4% of fetuses, respectively.

Conclusion: There is a wide scattering of measurements in fetuses with nuchal cord in comparison to the same fetuses in the absence of nuchal cord. This prevents accurate prediction of the true NT, although the largest and smallest of repeat measurements with nuchal cord can allow calculation of the highest and lowest possible risks, respectively. These facts must be taken into consideration in counseling patients.