Nuchal Translucency Thresholds in Prenatal Screening for Down Syndrome and Trisomy 18

Abstract

ObjectiveTo determine if nuchal translucency (NT) can be used as a first trimester triage marker in prenatal screening for Down syndrome and trisomy 18. MethodsData 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 β-hCG and NT. ResultsCombined prenatal screening was always positive for Down syndrome when NT thickness exceeded 4.0mm. 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 2.8mm, 3.0mm, and 3.4mm at 11, 12, and 13 weeks of gestation, respectively. In women over 42 years of age, the upper threshold value for NT was 1.8mm, 2.4mm, and 2.7mm 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. ConclusionIn 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.