Sequential screening (SS) is a test that can tell the chance that a pregnancy has Down syndrome (trisomy 21), trisomy 18, and open neural tube defects (ONTDs). The first part of the SS is generally performed in the first trimester and consists of a blood draw from mom and an ultrasound. Part two consists of a second blood draw from mom.
The first part of the sequential screen is similar to the first trimester screen (FTS), and the second part is similar to second trimester screening (STS). The first part of the sequential measures two hormones and proteins that are in mom’s blood that are coming from the pregnancy: pregnancy-associated plasma protein-A (PAPP-A), and human chorionic gonadotropin (hCG).
The results of the blood test is combined with the first trimester ultrasound, which measures the fetus to verify dating as well as pocket of fluid behind the baby’s neck called the nuchal translucency (NT). The lab uses a formula that includes the information from the blood work as well as the NT measurement to come up with a risk number. The second part of the sequential screen measures hCG again, as well as:
The lab uses information from the blood draws 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.
The first part of the sequential screen will usually give the chance for Down syndrome and trisomy 18. The results do not tell you ‘yes’ or ‘no’, but tell you the chance that the pregnancy has that condition. For example, the first part of the sequential screen may say that the chance for Down syndrome is 1 in 5,000, and the chance for trisomy 18 is 1 in 10,000. 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.
A low-risk, or negative, SS means the chances are reduced, but this test does not rule out any medical conditions. Similarly, a SS 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’ SS 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 SS 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 90-96% of pregnancies that have Down syndrome and trisomy 18 will have a positive SS, which means that approximately 4-10% will have low-risk SS 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 3-5% (depending on the lab).
If you’re on the fence about whether or not to pursue SS, 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.