First trimester screening (FTS) is a test that can tell the chances that a pregnancy has Down syndrome (trisomy 21) and trisomy 18 (some labs also offer screening for trisomy 13). FTS is generally performed between 11 and 14 weeks of the pregnancy, and consists of two parts: a blood draw from mom and an ultrasound. The blood draw measures two 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 first trimester ultrasound measures the fetus to verify dating, as well as pocket of fluid behind the baby’s neck called the nuchal translucency (NT). There should be some fluid in this area, but if the fluid measures larger than expected, it increases the chance for Down syndrome, trisomy 18, and other birth defects. Click here to read more about ultrasound.
The lab uses the measurements from the ultrasound and the blood draw, combined with information 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 first trimester screen result may say that the risk for Down syndrome is 1 in 5,000, and the risk for trisomy 18 is 1 in 10,000. This means that if there were 5,000 women that had the exact same test result as you, 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, FTS means the chances are reduced for these conditions, but this test does not rule any medical conditions out. Similarly, a FTS 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’ FTS 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 FTS 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 make sure to get more information and try to make sure you have all the necessary information to decide how to move forward. You may be offered further testing, including cell-free DNA, chorionic villus sampling (CVS), or amniocentesis. The choice to undergo further testing is yours, and is always voluntary.
A low-risk test result means the chance that the pregnancy has those conditions is reduced but not eliminated. Only about 80% of pregnancies that have Down syndrome and trisomy 18 will have a positive FTS, which means that 20% will have low-risk FTS 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 about 5%.
If you’re on the fence about whether or not to pursue FTS, it may be helpful to consider the following:
