Nuchal translucency
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Late in the first trimester, an echolucent area can be identified at the back of the neck of normal fetuses. Normally thin, it has been observed that an unusually thick translucency is sometimes associated with such abnormalities as trisomy 21 and other fetal malformations.
Between the 11th and end of the 13th week of gestation, the measurement of nuchal translucency is obtained with the fetus in sagittal section and a neutral position of the fetal head (neither hyperflexed nor extended, either of which can influence the nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from leading edge to leading edge. It is important to distinguish the nuchal lucency from the underlying amnionic membrane.
Normal thickness depends on the overall size of the fetus (CRL), but it should not exceed 3 mm at any gestational age. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality. Between 65 and 85% of trisomic fetuses will have a large nuchal thickness. Further, other, non-trisomic abnormalities may also demonstrate an enlarged nuchal transparency. This leaves the measurement of nuchal transparency as a potentially useful 1st trimester screening tool, particularly in combination with biochemical screening. Abnormal findings allow for early careful evaluation of chromosomes and possible structural defects on a targeted basis.
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