Normal range for trisomy 21 risk
Sep 13, 2022
Source: Cainiu Health
Dr. Chen Zhe
Introduction
There are two methods for assessing the risk of fetal trisomy 21 during pregnancy: Down syndrome screening and non-invasive DNA testing. These two tests use different methodologies, resulting in different risk estimates. In Down syndrome screening, the cutoff value for trisomy 21 risk is defined as 1:270; values below this threshold are considered low risk and normal, while values above it indicate high risk.
Trisomy is short for Trisomy 21 syndrome, also known as Down syndrome. Normally, humans are diploid. In 2013, the normal reference range for risk assessment was established—so what exactly is the normal value range for Trisomy 21 risk?
Normal Value Range for Trisomy 21 Risk
During pregnancy, there are two methods to assess fetal risk for Trisomy 21 syndrome: maternal serum screening (commonly called "Down syndrome screening") and non-invasive prenatal testing (NIPT) using DNA analysis. These two tests differ in methodology, resulting in different risk assessments. In Down syndrome screening, the cutoff value for Trisomy 21 risk is defined as 1:270. A result below this ratio indicates low risk (normal), while a result above it indicates high risk. For non-invasive DNA testing, the normal risk range for Trisomy 21 is between -3 and +3. Values within this range indicate low risk and are considered normal, whereas values outside this range suggest higher risk.
If test values are significantly elevated, it may indicate that the child has Down syndrome. For a definitive diagnosis, amniocentesis can be performed. Generally, newborn Down syndrome screening is not 100% accurate—it only estimates risk levels. The most accurate confirmation comes from diagnostic testing such as amniocentesis.
Pregnant women are advised to undergo Down syndrome screening between weeks 15 and 20 of gestation, with the optimal window being weeks 16 to 18. Testing during this period yields the most accurate results, with detection accuracy reaching 70% to 80%. We hope this information is helpful to you. Wishing you good health and happiness!
