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 NT above a CRL of 75 mm; for example, 1.9 mm versus 1.8 mm observed at 75-79 mm, and 2.0 mm versus 1.8 mm at 80-84 mm. The log-quadratic regression underestimated the median NT below a CRL of 45 mm at 1.03 mm versus 1.2 mm 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 log-linear regression appears to be a poor fit using local data, a log-sigmoid regression could be considered.