Nuchal translucency in normal fetus and its variation with increasing crown rump length (Crl) and gestational age

Abstract

Background: Nuchal translucency (NT) is the fluid collection behind the fetal neck which can be measured by ultrasound at 11-14 weeks of gestation. Increase in the nuchal translucency thickness is associated with various congenital anomalies. Objective: To study the relationship between nuchal translucency thickness, crown rump length and gestational age in normal fetus. Methods: Prospective analytical study conducted on 211 pregnant women from March 2011 to August 2012. Measurement of Nuchal translucency thickness and crown rump length was performed by ultrasound at 11-14 weeks of gestation. The relationship between nuchal translucency thickness, crown rump length and gestational age was studied by using linear regression analysis. Results: The mean CRL was 63.67+13.48mm (range 41.2-88mm) and mean NT thickness was 1.55+0.35mm (range 0.8-2.7mm), respectively. The median gestational age was 12.9 weeks. The regression equation which shows relation between median NT thickness and CRL was described as follows: expected NT thickness = 0.013CRL+0.725, (R2 = 0.258, p <0.001). There was increase in the incidence of NT thickness more than or equal to 2.5mm; 1.7% in fetus between 12-12.9 weeks of gestation to 15.1% in fetus between 14.0-14.9 weeks. Conclusion: Our study offers normative data of NT thickness in normal fetus, which can be used as reference to screen various chromosomal and congenital abnormalities between 11-14 weeks of gestation. NT thickness increased with increasing CRL and a false positive rate increases with increasing gestational age.