Nuchal Translucency (Trisomy 21 – Down syndrome)

Nuchal translucency (NT) is a transitory physiological accumulation of liquid in the fetus’s neck. It can be visualized by ultrasound after the ninth week. It can be viewed in the first trimester with transvaginal or transabdominal ultrasound as a hypoecogenic image less than 3 mm thick.

Nuchal translucency may be measured in all fetuses with a CRL (Crown-to-Rump Length) between 38 mm and 84 mm between weeks 10 and 14. An increased thickness of nuchal translucency is associated with an increased risk of chromosomal anomalies and syndromes.

NT thickening in the first trimester of pregnancy was first suggested as an earlier ultrasound marker of Trisomy 21 by L. Szabó in 1988.
Measuring Nuchal Translucency is a very simple procedure.

Recommendations

• Measurements may be performed either with transvaginal or transabdominal transducers, provided a longitudinal cut of the fetus is achieved.
• Measurement marks should be place to measure the maximum thickness of the subcutaneous translucency between the skin and the soft tissue covering the cervical spine.
• The fetus’s image should be enlarged so as to cover as much as the screen as possible.
• When the fetus is lying on his back, near the amniotic membrane, wait for the fetus to move so as to prevent confusion with the amniotic membrane.

The thickness of the nuchal translucency increases as the CRL does and therefore as the gestational age does. Chromosomal anomalies are congenital diseases relatively frequent which have an approximate prevalence of one out of 350 live births.

The risk of T-21 may be estimated more accurately taking in consideration all possible variables including gestational age, CRL, family history, and Nuchal Translucency thickness. Nuchal Translucency with ultrasound detects about 80% of fetuses with T-21.

Nuchal Translucency has been used for years during the first trimester as an early ultrasound marker of chromosomal disorders, with a diagnostic sensitivity of 80%~90% according to a number of authors. In T-21 the excessive accumulation of liquid may be the result of overexposure to of Type VI collagen encoded by a gene in chromosome 21. It is the earliest, most sensitive and specific marker for T-21.

Nuchal translucency has some characteristics regarded as basic to be accepted as a screening method:

• Easy detection.
• Low cost.
• Reproducible.
• Measurable.
• High sensitivity.

The optimal screening period is between weeks 10 and 14 after the last menstruation since in most cases it is a “transitory marker”; accordingly, normality in the second trimester does not imply absence of chromosomal disorder.

Taking into account possible cardiovascular factors, an increased TN in a chromosomally normal fetus may predict heart disorders.