When should Down syndrome screening be performed?

The Down syndrome screening (also known as the Down syndrome prenatal screening test) is a shortened term for the prenatal screening test for Down syndrome. It involves analyzing a blood sample from the pregnant woman to measure serum levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3). These biomarker concentrations are then combined with maternal factors—including age, weight, and gestational age—to estimate the risk of the fetus having Down syndrome (trisomy 21) or neural tube defects.

When Should Down Syndrome Screening Be Performed?

Most hospitals recommend performing the Down syndrome screening between weeks 15 and 20 of pregnancy. The test requires a peripheral blood draw; serum is separated from the blood sample and analyzed for AFP, hCG, and uE3 concentrations. These results are integrated with maternal parameters—including estimated date of delivery, body weight, age, and gestational age at the time of blood collection—to calculate the risk of delivering a baby with congenital abnormalities. However, it is important to note that Down syndrome screening is a risk assessment tool—not a diagnostic test. Therefore, results are reported only as “high risk,” “low risk,” or “intermediate (borderline) risk”; they cannot definitively confirm or rule out fetal structural anomalies. If the result indicates high risk or falls within the borderline range, further diagnostic testing—such as amniocentesis or non-invasive prenatal testing (NIPT)—is recommended to establish a definitive diagnosis.

Knowledge Expansion: What Conditions Does Down Syndrome Screening Primarily Detect?

1. Open Neural Tube Defects

Open neural tube defects include conditions such as meningocele and myelomeningocele. Down syndrome screening reports typically classify risk as low, high, or borderline. A high-risk result raises significant concern and warrants follow-up diagnostic testing—such as amniocentesis or NIPT—to confirm whether the fetus has any abnormalities.

2. Chromosomal Abnormalities

Down syndrome screening primarily assesses whether the fetus has chromosomal abnormalities, most commonly trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome). Early identification helps significantly reduce the risk of delivering a baby with congenital defects due to chromosomal abnormalities.

The above outlines the optimal timing for Down syndrome screening. We hope this information is helpful to you.