Nuchal translucency thresholds in prenatal screening for Down syndrome and trisomy 18.
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.


CONCLUSIONS
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.
Department of Obstetrics and Gynaecology, Hôpital Maisonneuve-Rosemont, University of Montreal, Montreal QC; Prenagen Inc., Laval QC.
Pierre
P
Miron
Warnex Medical Laboratories, Laval QC.
Yvan P
YP
Côté
Department of Social and Preventive Medicine, Faculty of Medicine, University of Montreal, Montreal QC.
Jean
J
Lambert
D000328:Adult; D002887:Chromosomes, Human, Pair 18; D004314:Down Syndrome / Q000175:diagnosis*; D005260:Female; D006801:Humans; D008423:Maternal Age; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D014314:Trisomy / Q000175:diagnosis*; D016216:Ultrasonography, Prenatal