How to assess the neutral position of the fetus for the crown-rump length measurement at the nuchal translucency scan

The objective of this study was to establish a simple and reproducible method for the assessment of the fetal head position when measuring crown-rump length (CRL) at the nuchal translucency scan. Two observers conducted a retrospective analysis of a consecutive series of 570 images of CRL collected by the French College of Fetal Echography (CFEF) national practice assessment program for the first-trimester scan. The images were deemed hyper-flexed if no fluid was visible between the chin and the chest of the fetus. The images were deemed hyper-extended if the angle between the palate and the CRL line was 90° or more. The images were deemed neutral if no hyper-extension nor hyper-flexion was observed. The proportion of agreement for a non-neutral position of the fetal head was 91.3% (kappa=0.80, 95% CI [0.75 to 0.86]). Images with a non-neutral position corresponded to poor CRL quality images according to the CFEF score (relative risk=4.2, 95% CI [2.9 to 6.1] for one observer and 4.9, 95% CI [3.3 to 7.2] for the other observer). Proportions of agreement for the hyper-flexion and for the hyper-extension were 94.6% (kappa=0.80, 95% CI [0.72 to 0.87]) and 96.8% (kappa=0.81, 95% CI [0.72 to 0.90]), respectively. No additional lines were drawn on the ultrasound images during the evaluation process. The proposed method for evaluating the extent to which the fetal head has a non-neutral position at measurement of the CRL appears both simple and reproducible.