Nuchal Translucency (NT) Screening
In the past, women 35 and older at the time of delivery were offered prenatal diagnosis with amniocentesis or chorionic villus sampling. Many were not happy about the invasive nature of these tests or the risk involved. Now there's an alternative for those who want to avoid invasive testing.
During the second and third trimesters of pregnancy, abnormal accumulation of fluid behind the fetal neck can be identified and classified. One type is known as nuchal cystic hygroma and another as nuchal edema. In approximately 75 percent of fetuses with cystic hygromas, there is a chromosomal abnormality. In approximately 95 percent of cases, the abnormality is Turner syndrome (a chromosomal defect that affects females, altering appearance, bone development and reproduction). When nuchal edema is seen during an ultrasound, chromosomal abnormalities are found in approximately one-third. And in approximately 75 percent of these cases, the abnormality is trisomy 21 or 18. Edema is also associated with fetal cardiovascular and pulmonary defects, skeletal problems, congenital infection, and metabolic and hematologic disorders. Therefore, when nuchal edema is seen, the outcome is not good, even when the chromosomes are normal.
In the first trimester, the term translucency is used because the prevalence of these abnormalities is related to the thickness of the area of translucency. Increased nuchal translucency is associated with trisomy 21, Turner syndrome, and other chromosomal abnormalities, as well as many fetal malformations and genetic syndromes.