Effect of deviation from the mid-sagittal plane on the measurement of fetal nuchal translucency

Screening for trisomy 21 by fetal nuchal translucency (NT) thickness at 11 to 13 + 6 weeks' gestation requires an appropriate examination of the NT. Errors in the assessment of NT may lead to an under- or overestimation of the NT and to incorrect patient-specific risks. In this study we aimed to examine the importance of the mid-sagittal section and whether the acquired plane of the head and face influences the measurement of the fetal NT thickness.

Sixty three-dimensional volumes of the fetal head and face in a mid-sagittal plane were acquired. NT thickness was firstly measured in the mid-sagittal plane according to the guidelines of The Fetal Medicine Foundation. The head was then rotated by steps of 5 degrees up to 25 degrees around the mid-point of the biparietal diameter and occipitofrontal diameter and NT was measured again. All six NT measurements were taken by the same operator, Operator A, who on completion of the assessment of the 60 volumes repeated all the measurements. The whole process was then repeated by Operator B. Both operators were blinded to each others' measurements.

In the true mid-sagittal plane, the mean NT was 1.9 mm and it was above the expected median in 72.5% of the measurements. At a deviation of 15 degrees , mean NT was 1.5 mm, and 36.3% of the measurements were above the expected median. At a deviation of 25 degrees , mean NT was reduced to 1.3 mm and 17.9% of the measurements were above the expected median.

Fetal NT thickness is greatest in the mid-sagittal plane. Increasing deviation away from the mid-sagittal plane results in progressive underestimation of the fetal NT thickness and so to a corresponding underestimation of the patient-specific risk.