NUCHAL TRANSLUCENCY MEASUREMENTS - FREQUENCY-DISTRIBUTION AND CHANGESWITH 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 prosp ective observational study Setting Department of obstetrics in a Londo n teaching hospital. Participants One thousand seven hundred and seven pregnant women (1685 singleton and 22 twin pregnancies) with chromoso mally 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 transv aginal sonography (TVS) if difficulty was encountered with TAS (5.1%). Nuchal translucency measurement was repeated in 136 fetuses: the firs t 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 t ranslucency (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 d emonstrate an increase in nuchal translucency measurement between 9 an d 12 weeks of gestation and then a decrease at 13-14 weeks. This relat ion is best modelled using the quadratic equation: log(10) (NT) = -1.0 3 + (0.0314 x CRL) - (2.20 x 10(-4) x CRL(2)). Longitudinal data confi rmed an increase of nuchal translucency measurements with increasing g estation. Conclusion Nuchal translucency measurements increase signifi cantly with increasing gestation. The use of a single threshold nuchal translucency measurement throughout the first trimester is inappropri ate. We propose reference ranges of nuchal translucency measurements w ith gestation, aiming to improve the performance of nuchal translucenc y screening for chromosomal abnormalities.