The estimation of median nuchal translucency values between 10 and 14 weeks of pregnancy

Nuchal translucency (NT) is a useful marker in antenatal screening for Down's syndrome in the late first trimester of pregnancy. NT measurements increase with increasing crown rump length (CRL) so multiple of the median (MoM) values are used to allow for this. Log-linear and log-quadratic regressions of NT in relation to CRL have previously been proposed to calculate MoM values. Using data on 288,079 women, these models were compared with a log-sigmoid regression. The log-linear regression overestimated the median NTabove a CRL of 75 mm; for example, 1.9mm versus 1.8mm observed at 75-79 mm, and 2.0mm versus 1.8mm at 80-84 mm. The log-quadratic regression underestimated the median NT below a CRL of 45mm at 1.03mm versus 1.2mm observed. The sigmoid regression provided the best overall fit to the data across the range of CRL values (40-84 mm) corresponding to gestational ages of 76 to 99 days. The differences between the three models are small. If a loglinear regression appears to be a poor fit using local data, a log-sigmoid regression could be considered.