What is the normal value for Down syndrome screening (Trisomy 21)?

The Down syndrome screening (also known as “Triple Test” or “Quad Screen”) is performed to identify fetuses at risk for Down syndrome. Down syndrome is a sporadic chromosomal disorder, making this screening highly advisable.

What is the normal value for trisomy 21 in Down syndrome screening?

Down syndrome screening is typically conducted between weeks 16 and 20 of pregnancy. A trisomy 21 risk score below 1/1000 is considered low-risk. If the result falls between 1/270 and 1/1000, the risk is classified as intermediate. A risk score greater than 1/270 indicates a high risk, significantly increasing the likelihood that the fetus has Down syndrome. Regardless of whether the screening result indicates an intermediate or high risk, further diagnostic testing—such as non-invasive prenatal testing (NIPT) or amniocentesis—is required for definitive diagnosis.

Down syndrome screening detects approximately 60–70% of affected fetuses. It is important to understand that this test estimates the probability of fetal Down syndrome but does not provide a definitive diagnosis. In other words, an elevated screening index suggests a higher chance of having a baby with Down syndrome, but it does not confirm that the fetus is affected. For example, women aged 35 years or older have a higher baseline risk of having a baby with Down syndrome, yet most such pregnancies still result in unaffected babies. Conversely, even a normal screening result cannot guarantee that the fetus is free of Down syndrome.

After becoming pregnant, it is essential to attend all scheduled prenatal check-ups to monitor your baby’s development closely. We hope this information is helpful. Wishing you a joyful life and good health!