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To investigate the existence of a relationship between maternal body mass, maternal ethnicity and maternal smoking status and nuchal translucency (NT) in the first trimester of pregnancy.NT measurements from 130 339 euploid, singleton pregnancies were converted to NT multiples of the median (MoM) and delta NT using expected medians determined using regression analysis. Relationships between maternal body mass index (BMI), maternal weight, maternal ethnicity and maternal smoking status and NT MoM and delta NT were examined.NT increased with gestational age. Uncorrected NT MoM and delta NT demonstrated small but significant positive relationships with either maternal BMI or maternal weight. Both NT MoM and delta NT were slightly, but significantly, increased in smokers compared to non-smokers and Afro-Caribbean compared to Caucasians, and slightly, but significantly, decreased in Asians compared to Caucasians.Although statistically significant, all the changes reported are likely to be too small to be relevant in terms of correcting in prenatal screening. Copyright © 2011 John Wiley & Sons, Ltd.