Establishment and application of a normal range for nuchal translucency across the first trimester.

Before chorionic villus sampling at 10-13 weeks' gestation, 453 women had the crown-rump length and nuchal translucency (NT) measured with transabdominal ultrasound. There were 19 aneuploid pregnancies (ten cases of trisomy 21, six of trisomy 18, one of 47 + marker, one 47,XXX, and one 45,X mosaic). Average NT was 1.7 mm (range 0-5 mm), correlating with the crown-rump length, but not maternal age. A static cut-off of 2.5 mm gave a false-positive rate of 1.3 per cent for crown-rump length between 30 and 35 mm, rising to 13 per cent in fetuses with a crown-rump length between 50 and 65 mm. This gave an overall false-positive rate of 5.5 per cent for a detection rate of 30 per cent for trisomy 21. Applying a dynamic action limit (95th centile), the false-positive rate remained at 5 per cent irrespective of the crown-rump length, detecting 30 per cent of trisomy 21 and 36.8 per cent of all aneuploidies. Raising the action limit to the 97.5th centile halved the false-positive rate (2.5 per cent), with no change in trisomy 21 detection and only a slight decrease in aneuploidy detection (31.6 per cent). Aneuploid fetuses showed normal first-trimester growth. NT increases with gestational age, making a dynamic action limit necessary to decrease the false-positive rate, while maintaining aneuploidy detection rates. Aneuploidy does not cause significant first-trimester growth retardation, enabling normal ranges for NT with crown-rump length to apply.