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Image: The profile of a fetus at 12 weeks. The term nuchal translucency (NT) describes the sonographic appearance of a collection of fluid in the back of the fetus’s neck, indicated here with a yellow arrow.

The term nuchal translucency describes the sonographic appearance of a collection of fluid, which is found in the back of a fetus’s neck. Between 11 and 13 weeks of pregnancy, in fetuses with chromosomal disorders or other fetal defects, the thickness of this collection will increase.

A chromosomal defect is an anomaly of the fetus’s karyotype and may occur without obvious fetal malformations.

A fetal malformation is an error that occurs during the phase of organ formation (amniocentesis is unable to help with this as it only analyses the chromosomes)

Chromosomal defects can be detected with amniocentesis / villocentesis, which are procedures used to study the fetal chromosomes but not the fetal malformations. This can only be detected with ultrasound scan.

All women have a risk of delivering a baby affected by a chromosomal anomaly or a congenital defect. (advantage/ disadvantage of prenatal diagnosis). 

 The risk of chromosomal anomalies increases with maternal age. However age alone is only one risk factor and compared to the individual risk it carries limited reliability.

 Detection rate based on maternal age alone is only 30%, while the individual risk can increase the detection rate to around 95%. More effective results (99%) can be obtained using a recent screening method, which consists in analyzing fractions of fetal cells in the maternal blood stream (NIPD) see "what changes for pregnant women".

 These results have led  many countries  to abolish in the near future free access to invasive procedures based on maternal age related risk. They will only be offered to high risk pregnancies based on nuchal translucency screening.

 In addition to indicating chromosomal anomalies, an increased nuchal translucency can also indicate fetal congenital defects, which are not detected with amniocentesis / villocentesis. Given that fetal congenital defects are more common than chromosomal, the ultrasound study of the fetus’s organs will become part of first trimester study. The main congenital malformation is cardiac.

The diagnosis of cardiac malformation in a fetus, can improve life expectancy for the affected newborn.

A wide-ranging Bibliography can be found here.

Last Edit: 05/08/2013 7:56pm