Influence of gravidity and foetal gender on the value of screening variables in the first trimester of pregnancy

Correlations between the biochemical variables in relation to gravidity and foetal gender can introduce a bias in the calculated risk of chromosome abnormalities. Differences in NT measurements with respect to foetal gender do not seem to be of clinical importance. NT is independent of gravidity so routine use of NT compensates for the influence of these maternal-foetal variables on the values of biochemical parameters. Hence, the bias in overall combined screening is small.