OBJECTIVE: To determine if nuchal translucency (NT) can be used as a first 
trimester triage marker in prenatal screening for Down syndrome and trisomy 18.
METHODS: Data from first trimester prenatal screening in 77 443 women were 
stratified by maternal and gestational ages. They were then analyzed to identify 
NT thresholds above or below which only positive (high-risk) or negative 
(low-risk) results were reported by a first trimester prenatal screening test 
combining PAPP-A, free beta-hCG and NT.
RESULTS: Combined prenatal screening was always positive for Down syndrome when 
NT thickness exceeded 4.0 mm. As women aged, this upper NT threshold value 
changed according to gestational age. In women aged 35 to 37 years, combined 
prenatal screening was always positive when NT exceeded 2.8 mm, 3.0 mm, and 3.4 
mm at 11, 12, and 13 weeks of gestation, respectively. In women over 42 years of 
age, the upper threshold value for NT was 1.8 mm, 2.4 mm, and 2.7 mm at 11, 12, 
and 13 weeks of gestation, respectively. In women less than 35 years of age, we 
identified lower threshold values below which combined prenatal screening for 
Down syndrome was always negative.
CONCLUSION: In prenatal screening for Down syndrome and trisomy 18, it is 
possible to identify NT threshold values above which biochemical screening 
provides no additional benefit. In pregnancies in which NT is above the 
established upper cut-offs, invasive prenatal screening can be offered without 
delay.