When is the optimal time to perform the Down syndrome screening test?

The Down syndrome screening (also known as “Tang screening”) involves drawing peripheral blood from the pregnant woman, isolating serum from this blood sample, and measuring the concentrations of maternal serum alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3). These measurements are then combined with clinical parameters—including the mother’s estimated due date, weight, age, and gestational age at the time of blood draw—to calculate the risk of delivering a fetus with congenital abnormalities. So, when is the optimal timing for undergoing Tang screening? The following section addresses this question.

Optimal Timing for Tang Screening

Typically, the optimal window for Tang screening is between 15 and 20 weeks of gestation. This test entails drawing peripheral blood, isolating serum, and measuring maternal serum levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3); these values are then integrated with maternal factors—including estimated due date, body weight, age, and gestational age at sampling—to estimate the risk of fetal congenital anomalies. However, it is important to note that Tang screening is a risk-assessment tool—not a diagnostic test. Therefore, results are reported only as “high-risk,” “low-risk,” or “intermediate-risk”; they cannot definitively confirm or rule out fetal structural or chromosomal abnormalities. If results indicate high risk or fall within the borderline range, further diagnostic testing—such as amniocentesis or non-invasive prenatal testing (NIPT)—is recommended to establish a definitive diagnosis. Additionally, pregnant women are advised to prioritize adequate rest and maintain balanced nutrition throughout pregnancy.

Knowledge Extension: What Does Tang Screening Primarily Detect?

1. Chromosomal Abnormalities

Tang screening primarily assesses whether the fetus has chromosomal abnormalities—most commonly trisomy 18 and trisomy 21 (Down syndrome). Early identification helps significantly reduce the likelihood of delivering a baby with chromosomal disorder–related congenital defects.

2. Open Neural Tube Defects (ONTDs)

Open neural tube defects include conditions such as meningomyelocele. Tang screening reports classify risk as “low,” “high,” or “intermediate.” A high-risk result indicates significant suspicion for abnormalities, warranting follow-up diagnostic procedures—such as amniocentesis or non-invasive prenatal DNA testing (NIPT)—to clarify whether fetal anomalies are present.

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