Second trimester screening (STS), also called quad screen or triple screen, is a test that can tell the chances that a pregnancy has Down syndrome (trisomy 21), trisomy 18, and open neural tube defects (ONTDs). STS is generally done between 15 and 22 weeks of the pregnancy, and consists of a blood draw taken from mom. The triple screen measures three hormones and proteins that are in mom’s blood that are coming from the pregnancy: human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and alpha-fetoprotein (AFP).
The quad screen analyzes the three previous substances, as well as inhibin-A.
The lab uses information from the blood draw and about the mom (age, weight, ethnicity, etc) to calculate a risk number. The results will be broken down by each condition that was screened for. For example, a second trimester screen result may say that the chance for Down syndrome is 1 in 5,000, the chance for trisomy 18 is 1 in 10,000, and the chance for ONTDs is 1 in 3,000. This means that if there were 5,000 women that had the exact same test result, only 1 would have a child with Down syndrome. Likewise, if there were 10,000 women with the exact same test result, only 1 would have a child with trisomy 18. And if there were 3,000 women with the exact same test results, only 1 would have a child with an ONTD.
A low-risk, or negative, STS means the chances are reduced, but this test does not rule out any medical conditions. Similarly, a STS result that shows an increased risk does not mean that the pregnancy has that condition. If your results show an elevated risk, you may be offered further testing, such as a diagnostic prenatal test. It’s important to remember that most people who have a ‘positive’ or ‘high-risk’ STS will go on to have healthy babies.
People choose either to do or not to do prenatal genetic screening for many reasons. It is important to remember that this testing is voluntary and is not required.
If you had a STS done with your primary OB-GYN, they may refer you to a high-risk pregnancy doctor called a perinatologist. You may also be referred to meet with a genetic counselor. These visits are designed to gather more information, but also to make sure you have a clear understanding of everything that is going on. It is also important during these visits to talk about the risks, benefits, and limitations of different options for further testing, including cell-free DNA or amniocentesis. The choice of whether or not to undergo further testing is yours, and further testing is always voluntary.
A low-risk test result means the chance that the pregnancy has the conditions that were screened for is reduced, but not eliminated. Depending on the lab, between 75-85% of pregnancies that have Down syndrome and trisomy 18 will have a positive STS, which means that approximately 15-25% will have low-risk STS results. The chance that a pregnancy that does not have Down syndrome or trisomy 18 will have a high-risk test result, or a false positive, is usually between 5-8% (depending on the lab).
If you’re on the fence about whether or not to pursue STS, it may be helpful to consider the following:
It is important to talk with your medical provider or a genetic counselor if you have further questions or concerns.
Click here to learn more about scheduling a genetic counseling appointment for pregnancy-related questions.