What’s the Difference Between First-Trimester and Second-Trimester Down Syndrome Screening?

First-trimester and second-trimester screening for Down syndrome—also known as trisomy 21—are two distinct prenatal risk assessment strategies, each with unique timing, methodology, and performance characteristics.

First-trimester screening is typically performed between 11 weeks and 13 weeks + 6 days of gestation. It combines maternal serum testing—measuring pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (free β-hCG)—with a specialized ultrasound assessment known as the nuchal translucency (NT) scan. The NT measurement evaluates the fluid-filled space at the back of the fetal neck; increased thickness is associated with an elevated risk of chromosomal abnormalities, including Down syndrome. When integrated with biochemical markers and maternal factors (such as age, weight, and gestational age), first-trimester screening achieves a detection rate of approximately 82–87% for Down syndrome, with a false-positive rate of around 5%.

In contrast, second-trimester screening—often referred to as the “quad screen”—is conducted between 15 weeks and 22 weeks of gestation. It analyzes four maternal serum analytes: alpha-fetoprotein (AFP), unconjugated estriol (uE3), inhibin A, and total or free β-hCG. This combination improves sensitivity over earlier two- or three-marker tests. The quad screen detects about 80% of fetuses with Down syndrome, with a comparable false-positive rate of roughly 5%. While it does not include an ultrasound component, some clinicians perform a targeted anomaly scan concurrently to assess structural development and identify soft markers that may further inform risk.

Critically, neither test is diagnostic. Both are risk-assessment tools designed to identify pregnancies warranting confirmatory testing—such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. Cell-free DNA (cfDNA) screening, though increasingly used as a primary or contingent screening option, remains a separate modality with higher sensitivity and specificity but does not replace the clinical utility of traditional serum and ultrasound-based assessments in all populations.

Choice between first- and second-trimester screening depends on multiple factors—including gestational age at initial prenatal visit, local resource availability, patient preference, and whether combined or sequential protocols are offered. Integrated or contingent screening models—which combine results from both trimesters—can achieve detection rates exceeding 95%, albeit with slightly higher false-positive rates and longer wait times for final risk assessment.