What does the Down syndrome screening test examine?

The Down syndrome screening test (often referred to as “Tang screening”) is a scientifically validated prenatal assessment method designed to evaluate the risk of fetal abnormalities. It involves analyzing maternal blood to measure serum levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3), combined with maternal factors such as age, weight, and gestational age, to estimate the risk of fetal Down syndrome (trisomy 21) and neural tube defects.

What Does the Down Syndrome Screening Test Assess?

The Down syndrome screening test—commonly abbreviated as “Tang screening”—is a prenatal screening tool for detecting the risk of Down syndrome. It assesses the likelihood that the fetus has Down syndrome by analyzing maternal blood. If the screening results indicate an elevated risk, further diagnostic testing—such as amniocentesis or chorionic villus sampling (CVS)—is recommended to confirm the diagnosis. The optimal window for Tang screening is between 15 and 20 weeks of gestation, with the ideal timing being 16–18 weeks. However, women aged 35 years or older, those with a family history of Down syndrome, or those who have previously given birth to a child with Down syndrome are considered high-risk and should proceed directly to diagnostic testing (e.g., amniocentesis or CVS) rather than undergoing the initial screening.

Knowledge Extension: Key Points Regarding Tang Screening Results

1. Tang screening only estimates the probability that the fetus has Down syndrome; it cannot definitively diagnose the condition. Elevated biochemical markers suggest a higher likelihood of Down syndrome, but do not confirm the presence of the disorder.

2. Similarly, while advanced maternal age (≥35 years) increases the statistical risk of having a baby with Down syndrome, it does not guarantee that the fetus is affected. Conversely, normal screening results do not completely rule out the possibility of Down syndrome. Pregnant individuals identified as high-risk based on screening should undergo confirmatory diagnostic testing—either amniocentesis or CVS—to evaluate fetal chromosomal status and definitively determine whether Down syndrome is present.

The above provides an overview of what the Tang screening test evaluates. We hope this information is helpful to you.