This is normally less than 2.5mm and when seen increased (greater than 2.5mm, see image on the right) may indicate the baby has Down syndrome or may indicate another chromosomal abnormality. The nuchal translucency test will also check whether your baby has a visible nasal bone.In the past few years it has been seen that approximately 3 in 4 babies with Down's syndrome do not have a visible nasal bone at the time of the first trimester screening test.
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First trimester ultrasonic scans may show 'soft' markers for chromosomal abnormalities, such as an increased fetal nuchal translucency (back of the neck) to enable detection of Down's syndrome fetuses.
The test is performed between 11 weeks and 13 weeks 6 days into your pregnancy.
At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will look for any major problems.
This is a routine ultrasound examination done at 10 to 14 weeks of gestation.