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A nuchal scan is an ultrasound, or a sonographic prenatal screening scan, performed by your doctor during weeks 11-13 of pregnancy. After 13 weeks and six days, it is no longer an option. It is also sometimes called the nuchal translucency screening (NT), or the nuchal fold scan.
Prenatal screening is used to determine whether any abnormalities or potential birth defects are present. The nuchal scan is often a noninvasive first step in searching for abnormalities. If the nuchal scan shows some cause for concern, a more invasive procedure, such as amniocentesis or a chorionic villus sampling (CVS), may be performed.
It is best to have the nuchal scan completed as soon as possible because a CVS must be performed between 11 and 12 weeks. The amniocentesis will take place at 15 to 18 weeks. As always, it is best to allow plenty of time to complete any further testing.
A nuchal scan measures the translucent space in the nuchal fold at the back of your baby's neck. A higher-than-normal accumulation of fluid in this space can be an indication that the baby is at a higher risk of being born with Down syndrome, also called Down's syndrome. The crown-to-rump length of the baby is measured to be sure of an accurate age in weeks. The age of the baby is then compared to the thickness of the nuchal fold which provides a more exact measurement.
The age of the mother is also taken into account as the risk of having a baby with Down syndrome increases with the age of the mother. At age 20, your odds of having a baby with Down syndrome are approximately 1 in 1,000, but at age 35, your risk is approximately 1 in 400. The likelihood continues to increase past age 35.
The scan does not determine if the baby has Down syndrome; it only indicates a higher risk factor, which can then be followed up with diagnostic testing. A nuchal scan is accurate at detecting potential abnormalities 70-80% of the time, but of course, there is always the possibility for false positives and false negatives. Any concerns you may have should always be discussed with your doctor.
Because the nuchal translucency scan is a relatively new procedure, requiring new equipment and new training, it may be more difficult to find a doctor that is able to perform the ultrasound. Your physician should be able to direct you to a medical center that is able to help. Another option is to contact your insurance company, as they will be able to advise you more specifically.
CVS can be performed between 11 and 15 weeks. The optimal week for the nuchal scan is 12 weeks which still allows time for the CVS afterward.
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