Establishment and Application of a Normal Range for Nuchal Translucency Across the First Trimester

Abstract

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,%, 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.

Variaton of nuchal translucency with increasing crown rump length and gestational age in normal singleton pregnancies

“…In our study the mean NT thickness in normal fetuses was 1.5 mm, as compared to a study in Taiwanese population, where the mean NT thickness was 1.7 mm (11) . We found that the incidence of NT thickness greater than or equal to 2.5 mm was 2.8% and this observation was in accordance with previous studies (2,8) .The incidence of NT thickness greater than or equal to 2.5 mm in normal fetuses was 2.5% at 11-11.6 weeks at gestation and increased to 5.6% at 13-13.6 weeks, in our study. Other studies have reported that the incidence of NT thickness greater than or equal to 2.5 mm in normal fetuses increased from 1.3% at CRL 30-39 mm to 13.2% at CRL 60-69 mm (8) .…”

“…(4,7,8) .Therefore, in screening chromosomal defects, the use for a fixed cut-off in NT thickness is inappropriate and each measurement should be examined according to the CRL. (9) Multiple of median values to express the relationship between NT measurements and gestational age can be used (10).…”

Screening of Maternal Serum for Fetal Down's Syndrome in the First Trimester

“…The median nuchal-translucency thickness ranged from 1.0 mm to 4.0 mm at the different centers (the expected median value is approximately 1.5 mm and is known to increase with gestational age). 23 The overall rate of detection of Down's syndrome (based on a criterion of nuchaltranslucency measurements above the 95th percentile for each center) was 31 percent, with a 5 percent false positive rate. The variability among centers in median values and in the ratios of the 95th percentile to the 50th percentile, coupled with their vary-ing ability to obtain the measurements successfully, suggests that the performance of this indicator in our study might not accurately reflect its long-term performance at individual centers.…”

Hyperclarté nucale et hygroma cervical au premier trimestre de la grossesse : diagnostic prénatal et devenir néonatal

“…Elle est corrélée à l'âge gestationnel chez un foetus normal [13,14]. Après 14 SA, il existe une disparition spontanée de cette hyperclarté nucale [15], phénomène surtout visible chez le foetus à caryotype normal, alors qu'en cas d'aneuploïdies, elle tend à augmenter [16].…”