Nuchal Translucency Screening
What is nuchal translucency?Ultrasound done between 11 weeks 4 days and 14 weeks 3 days will measure the fluid accumulation behind the neck of the baby, called the nuchal translucency (NT). Extra fluid, (an increased NT) can be a sign of Down syndrome, trisomy 18 or trisomy 13.
Why is it called "screening"?Screening only provides an estimate of risk. A positive result, indicating an increased risk, does not mean the baby has a problem. Diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis, is the only way to determine with certainty if a baby has Down syndrome, trisomy 18 or trisomy 13. A negative result, indicating a lower risk, does not eliminate the risk that the baby will have Down syndrome, trisomy 18 or trisomy 13.
First trimester screening detects about 85% of pregnancies in which the baby has Down syndrome and 91% of pregnancies in which the baby has trisomy 18.
Who should be offered this screening?All pregnant women should be offered NT screening. In May, 2009 the California Genetic Disease Branch incorporated 1st trimester screening into their existing 2nd trimester screening (AFP) program. Women of all ages must be given the option of participating.
Any woman, regardless of age may choose to decline screening or may choose to have diagnostic testing such as CVS or amniocentesis. If they desire CVS or amniocentesis, they must first meet with a genetic counselor to review their risks and options.
What are the advantages of first trimester screening?First trimester screening is done earlier in the pregnancy and can give couples important information as early as possible.
First trimester screen results can be combined with second trimester screen results to detect more real cases of Down syndrome and trisomy 18, at the same time avoiding too many false positive results.
Increased nuchal translucency has also been associated with other fetal anomalies such as cardiac defects. A second trimester fetal anatomy ultrasound and echocardiogram are recommended if the NT is increased.
What are the limitations of first trimester screening?Risk figures can only be calculated for Down syndrome and trisomy 18/trisomy 13.
First trimester screening does not assess a risk for neural tube defects. All women will continue to be offered second trimester screening for neural tube defects through the state program.
How long do results take?Patients are encouraged to have the blood test portion of the screen drawn at least one week prior to their NT ultrasound. This allows the physician performing the NT to access her blood results and give her the individual risk for Down syndrome the day of the NT ultrasound.
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