Increased nuchal translucency is defined as an abnormal collection of fluid under the skin at the back of the fetus' neck. In the majority of cases, the fluid disappears and has no effect on the baby. However, abnormal nuchal translucency is an ultrasound "marker", which indicates increased risk for certain problems.
Approximately 20-60% of babies who demonstrate an increased nuchal translucency on ultrasound may have an associated chromosome abnormality, particularly Down syndrome or Turner syndrome. Chromosomes are the inherited structures in the cells of the body. There should be 46 chromosomes in each cell, arranged into 23 pairs. Chromosome abnormalities involving a missing or extra chromosome are not usually inherited and are not caused by an exposure during pregnancy. Instead, they are caused by random events in cell division at the time of conception and can occur in anyone's pregnancy. Individuals with Down syndrome have an extra #21 chromosome, thus three rather than two copies of chromosome 21. It is this extra genetic material that causes the features of Down syndrome, including intellectual disability, a characteristic facial appearance, and other health problems. Turner syndrome is caused by a missing sex chromosome. Individuals with Turner syndrome are always female, are typically short in stature and infertile, and may have heart and/or kidney problems.
Nuchal translucency measurement can be combined with maternal serum information for First Trimester Screening.
Prenatal diagnosis by CVS or amniocentesis is available to test for these chromosome abnormalities during pregnancy.
Increased nuchal translucency has also been associated with an approximate 5-20% risk for a defect of the heart. A detailed ultrasound performed at 18-20 weeks can help identify these birth defects. A specialized ultrasound, called an echocardiogram, is used to look specifically at the structure of the heart.
Babies with increased nuchal translucency are also at increased risk to be affected with certain genetic syndromes or skeletal disorders caused by gene changes (mutations). Genes are the units of heredity that make up the chromosomes. Genes are not visible when looking at a picture of chromosomes. However, DNA testing is available for certain genetic syndromes caused by gene mutations. If there are indications within the family history or by ultrasound that implicate a certain genetic syndrome, DNA testing for that syndrome may by available prenatally through CVS or amniocentesis.
It is important to remember that excess increased nuchal translucency may be a normal variant in a baby and cause no health problems.