Objective To evaluate screening for chromosomal defects by a combination of fetal nuchal translucency thickness and maternal age.
Design A prospective multicentre screening study where fetal nuchal translucency thickness was measured at 10 to 14 weeks of gestation.
Subjects 20,804 women with singleton pregnancies screened at 10 to 14 weeks of gestation from 1 September 1992 to 28 October 1994.
Main outcome measures Trisomy 21 and other chromosomal defects identified by increased nuchal translucency thickness and by a combination of nuchal translucency thickness and maternal age.
Results In normal fetuses nuchal translucency thickness increased significantly with crown-rump length. The nuchal translucency was above the 95th centile in 77% (66 of 86) of fetuses with trisomy 21 and in 78% (61 of 78) of those with other chromosomal defects. On the basis of the distribution of nuchal translucency measurements in normal fetuses and those with trisomy 21, a new method of screening is proposed which involves assessment of individual risk based on the combination of fetal nuchal translucency, crown rump length and maternal age. The minimum risk was 1/100 in 4.9% of the normal pregnancies, in 80% of those with trisomy 21 and in 77% of those with other chromosomal defects.
Conclusion Screening for fetal trisomy 21 can be carried out effectively during the first trimester of pregnancy.
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