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The advantage of nuchal scanning over the previous use of just biochemical blood profiling is mainly the reduction in false positive rates.
When screening is positive, chorionic villus sampling (CVS) or amniocentesis testing is required to confirm the presence of a genetic abnormality.
A nuchal scan or nuchal translucency (NT) scan/procedure is a sonographic prenatal screening scan (ultrasound) to detect cardiovascular abnormalities in a fetus, though altered extracellular matrix composition and limited lymphatic drainage can also be detected.
Since chromosomal abnormalities can result in impaired cardiovascular development, a nuchal translucency scan is used as a screening, rather than diagnostic, tool for conditions such as Down syndrome, and non-chromosomal abnormalities, including the genetic Di George syndrome and non-genetic Body-stalk anomaly.
So after this time any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by nuchal scanning.
The buildup in fluid is due to a blockage of fluid in the developing fetal lymphatic system.
Until recently, the only reliable ways to determine if the fetus has a chromosomal abnormality was to have an invasive test such as amniocentesis or chorionic villus sampling, but such tests carry a risk of causing a miscarriage estimated variously as ranging between 1% Most women, especially those with a low risk of having a child with Down syndrome, may wish to avoid the risk to the fetus and the discomfort of invasive testing.
An increased nuchal translucency increases the probability that the fetus will be affected by a chromosomal abnormality, congenital cardiac defects, or intrauterine fetal demise.
It is important to distinguish the nuchal lucency from the underlying amniotic membrane.
Normal thickness depends on the crown-rump length (CRL) of the fetus.
As of 2015, there are five commercial versions of this screen (called cell-free fetal DNA screening) available in the United States.
Blood testing is also used to look for abnormal levels of alphafetoprotein or hormones.