Fetal nuchal translucency screening in 12495 pregnancies in Sardinia

Abstract

Objective: To examine the distribution of fetal nuchal translucency thickness in normal and chromosomally abnormal fetuses in Sardinia and to determine the effectiveness of screening by a combination of fetal nuchal translucency and maternal age.

Methods: Fetal nuchal translucency thickness and crown-rump length were measured at 10-14 weeks of gestation in 12 495 pregnancies. A reference range of fetal nuchal translucency thickness for crown-rump length was determined from the 10 001 singleton pregnancies with known normal pregnancy outcome. The median nuchal translucency thickness for crown-rump length was determined by regression analysis of the calculated median values of nuchal translucency thickness for each 0.1 mm interval in crown-rump length. The proportions of unaffected fetuses and those with trisomy 21 or other chromosomal defects with nuchal translucency thickness > 1.5 and 2.0 multiples of the regressed normal median for crown-rump length were calculated. The distribution of estimated risks based on maternal age and fetal nuchal translucency thickness according to The Fetal Medicine Foundation software were also determined and the sensitivity and false-positive rates were calculated.

Results: In the 10 001 normal pregnancies, the median fetal nuchal translucency thickness increased with crown-rump length (median nuchal translucency thickness = 0.3496 + 0.018 x crown-rump length) (r2 = 0.4411). In the singleton pregnancies, there were 64 fetuses with trisomy 21 and 46 with other chromosomal defects. The fetal nuchal translucency thickness was > 1.5 multiples of the median in 510 (5%) of the normal fetuses, in 52 (81%) of the trisomy 21 fetuses and in 33 (72%) of those with other chromosomal defects. The respective values for nuchal translucency thickness > 2.0 multiples of the median were 195 (2%), 41 (64%) and 32 (70%). In 184 multiple pregnancies, there were four fetuses with chromosomal abnormalities and in three of these the nuchal translucency thickness was > 1.5 multiples of the median. Screening by a combination of maternal age and fetal nuchal translucency thickness with a risk cut-off of 1 in 300 identified 90% of trisomy 21 pregnancies and 85% of all other chromosomal defects for a false-positive rate of 9%.

Conclusion: Screening for chromosomal defects by measurement of nuchal translucency thickness identifies 80% of fetuses with trisomy 21 for a false-positive rate of 5%. In our population with a median maternal age of 33 years, screening by a combination of maternal age and fetal nuchal translucency thickness with a risk cut-off of 1 in 300 identified 90% of trisomy 21 pregnancies for a false-positive rate of 9%.