First trimester screening includes an ultrasound exam to measure the size of the clear space in the tissue at the back of a baby's neck (nuchal translucency). In Down syndrome, the nuchal translucency measurement is abnormally large — as shown on the left in the ultrasound image of an 11-week fetus. For comparison, the ultrasound image on the right shows an 11-week fetus with a normal nuchal translucency measurement.
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