The Quad Screen is a non-invasive prenatal blood test offered during the second trimester. It is a screening tool that assesses the likelihood of a fetus having certain genetic conditions or birth defects, rather than providing a definitive diagnosis. The test measures the levels of four specific substances that pass from the fetus and placenta into the mother’s bloodstream, helping health care providers determine if further testing is needed.
Why the Quad Screen is Offered
The Quad Screen evaluates the risk for specific congenital disorders. It is typically performed between the 15th and 22nd weeks of pregnancy, with the 16th to 18th weeks being the most accurate window. The test screens for the increased chance of the fetus having Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Neural Tube Defects (NTDs).
NTDs are structural birth defects involving the brain and spinal cord, such as spina bifida. Trisomy 21 and Trisomy 18 are chromosomal conditions that cause intellectual disability and developmental delays.
The Four Markers Measured
The Quad Screen is named for the four biochemical markers it analyzes in the maternal serum. These substances are produced by the developing fetus and the placenta. The four markers are:
- Alpha-fetoprotein (AFP)
- Human chorionic gonadotropin (hCG)
- Unconjugated estriol (uE3)
- Inhibin A
Alpha-fetoprotein (AFP) is a protein made by the fetal liver and yolk sac. Elevated AFP levels are associated with an increased risk of open Neural Tube Defects, while low levels are often seen in pregnancies affected by Down syndrome.
Human chorionic gonadotropin (hCG) is a placental hormone; levels are typically higher than average when the fetus has Down syndrome. Unconjugated estriol (uE3) is a form of estrogen produced by the fetus and the placenta. Low levels of uE3 are commonly observed in cases of both Down syndrome and Trisomy 18.
Inhibin A is a placental hormone. High levels of Inhibin A are associated with an increased risk for Down syndrome. The levels of these four substances are combined with factors like the mother’s age, weight, and gestational age to calculate a personalized risk score.
Interpreting Your Screening Results
The Quad Screen provides a probability, not a diagnosis. A “low-risk” or negative result suggests the chance of the fetus having one of the screened conditions is reduced, but it does not guarantee the absence of a disorder.
A “high-risk” or positive result indicates that the calculated chance of a condition is higher than a predetermined cutoff. For example, a 1 in 100 chance for Down syndrome is considered high-risk. This result prompts a discussion about further testing, as it does not mean the fetus has the condition.
The test has a known rate of false positives, occurring when the test indicates high risk but the fetus is unaffected. This is often caused by inaccurate dating of the pregnancy. Conversely, a false negative occurs when the result is low-risk, but the condition is present.
Following a high-risk result, follow-up options are typically offered. These may include a detailed ultrasound to check for physical markers and confirm gestational age. Diagnostic tests, such as amniocentesis, may also be offered to provide a definitive answer by analyzing fetal cells.