Definition of "Nuchal scan"

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Nuchal scan (aka nuchal translucency, nuchal fold) is an U/S prenatal screen assessing the quantity of fluid collecting within the nape of the fetal neck.

Method
  • Examined on the early morphology conducted at the end of trimester 1 (weeks 0-12), carried out at 11-13 weeks gestation. The scan is obtained with the fetus in saggital section, and a neutral position of the fetal head (neither hyperflexed nor extended, as it can influence nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from edge to edge. It is important to distinguish the nuchal lucency, from the underlying amniotic membrane
Purpose
  • Increased thickness measurements are associated with:
    • Higher chances for chromosomal conditions, e.g. Down syndrome in a fetus, particularly for older women who have higher risks of such pregnancies, tending to have an increased amount of fluid around the neck. High definition imaging may also detect other less common chromosomal abnormalities
    • Also associated with congenital heart defect
  • Confirms both the accuracy of the pregnancy dates, and fetal viability
Pathophysiology
  • The fluid seen as translucency, can be edematous skin at the back of the neck, or dilated lymphatic sacs filled with fluid, due to altered normal embryological connections
  • Nuchal translucency however, is ONLY useful to measure between 11-14 weeks gestation, when the fetal lymphatic system is developing, and the peripheral resistance of the placenta is high. After 14 weeks, the lymphatic system is likely to have developed sufficiently to drain away any excess fluid, and changes to the placental circulation will result in a drop in peripheral resistance. Fluid accumulated will thus correct itself after this time
Interpretation
  • If the screening is POSITIVE, it should be followed up with CVS (earlier) or amniocentesis (later), to confirm the Dx. However, because it carries a small risk of miscarriage, SCREENING should be used to minimize miscarriage
Epidemiology
  • The nuchal scan 1st came into widespread use in 2003
See also

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Definition of Nuchal scan | Autoprac


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