Gestational age standardized nuchal thickness values for estimating mid‐trimester Down's syndrome risk

Objective: Our aim was to develop gestational age standardized indices of fetal nuchal thickening. In addition, we wanted to develop a method for combining nuchal thickness data with maternal age for calculating individual Down's syndrome risk. Methods: Nuchal thickness was measured prospectively in pregnancies undergoing genetic amniocentesis. A regression equation for expected median nuchal thickness based on the biparietal diameter (BPD) was developed. Nuchal thickness values were expressed as multiples of the median (MoM). Additionally, a new parameter, percentage increase in nuchal thickness (PIN) (measured minus expected nuchal thickness) ×100/expected nuchal thickness, was used. Receiver operator characteristics curves for Down's syndrome detection based on nuchal thickness values expressed as MoM, PIN, and in mm were compared. Log10 transformation of MoM data resulted in a Gaussian distribution, and the Down's syndrome likelihood ratios were calculated based on the heights of the Gaussian curves. Likelihood ratios were also calculated based on PIN values. The screening efficiency of maternal age alone was compared to age plus MoM, and age plus PIN values by multiplying age‐related risk by the likelihood ratio corresponding to the given nuchal thickness MoM or PIN values. Results: There were 3,574 chromosomally normal and 50 Down's syndrome fetuses in the study. Both PIN and MoM values for nuchal thickness were closely correlated (R = 1.00, P