Nuchal translucency measurements: Frequency distribution and changes with gestation in a general population

Objective To investigate nuchal translucency variation with crown-rump length (CRL) in the first trimester in normal fetuses. Design A prospective observational study Setting Department of obstetrics in a London teaching hospital. Participants One thousand seven hundred and seven pregnant women (1685 singleton and 22 twin pregnancies) with chromosomally normal fetuses between 9(+0) and 14(+6) weeks of gestation were recruited sequentially from a dating scan clinic. Nuchal translucency was measured in 94.9% by transabdominal sonography (TAS) and by transvaginal sonography (TVS) if difficulty was encountered with TAS (5.1%). Nuchal translucency measurement was repeated in 136 fetuses: the first scan between between 9(+0)-11(+6) weeks and the second over 12 weeks of gestation. Results Of this general population, 4.2% had a nuchal translucency (NT) measurement greater than or equal to 2.5 mm, but this proportion varied significantly between gestational age groups (P = 0.011). The repeatability coefficients for NT measurements were 0.44 mm and 0.23 mm for TAS and TVS, respectively. The cross-sectional data demonstrate an increase in nuchal translucency measurement between 9 and 12 weeks of gestation and then a decrease at 13-14 weeks. This relation is best modelled using the quadratic equation: log(10) (NT) = -1.03 + (0.0314 x CRL) - (2.20 x 10(-4) x CRL(2)). Longitudinal data confirmed an increase of nuchal translucency measurements with increasing gestation. Conclusion Nuchal translucency measurements increase significantly with increasing gestation. The use of a single threshold nuchal translucency measurement throughout the first trimester is inappropriate. We propose reference ranges of nuchal translucency measurements with gestation, aiming to improve the performance of nuchal translucency screening for chromosomal abnormalities.