The American Pregnancy website claims Down syndrome (Trisomy 21) is the most common birth defect in the United States. Caused by an extra copy of chromosome number 21, there are a handful of symptoms usually identified during the course of routine prenatal care that can lead a doctor to suspect Down syndrome. There are only two tests, an amniocentesis and a chorionic villus sampling (CVS), which can definitively diagnose Down syndrome. Because of the risks associated with these tests, it is best to consult with a doctor to consider the diagnostic options.
Screening Tests
The results of a triple screen or quad marker screen can clue a doctor into the possibility of Down syndrome. These tests calculate the estimated risk of a baby having the defect, and an abnormal result means that the probability is higher. Both tests are most accurate when performed between the 16th and 18th week of pregnancy, according to the Family Doctor website, and many doctors will explain further testing options with abnormal results.
Nuchal Folds
There are several characteristics, often referred to as "ultrasonic soft markers," that can be identified via ultrasound and that can indicate Down syndrome. The most common soft marker for which trained sonographers look is nuchal folds thickness, or nuchal translucency. Nuchal translucency refers to an area of fluid behind the neck of the fetus, which leads to thickening of the baby's neck skin folds. Measured in fractions of millimeters, the Your Ultrasound website writes that researches have identified a correlation between increased nuchal translucency and Down syndrome.
Other Soft Markers
There are a handful of other soft markers that can be seen during a second trimester sonogram. These are differences also seen in a full-term baby with Down syndrome, according to the Your Ultrasound website, and include a shorter nose, lower-set ears, shorter limbs and some organ abnormalities.
Ultrasound Accuracy
There is a significant error rate that may be as high as 50 percent when it comes to a Down syndrome diagnoses based solely on ultrasound, according to the Your Ultrasound website. This is why most prenatal healthcare specialists will recommend screening tests together with an ultrasound. Should both of these raise some red flags, then a patient and doctor will weigh the benefits of conclusive diagnostic tests like CVS or amniocentesis.
