First trimester screening (FTS)
To the Point: First trimester screening (FTS) can tell you the chances your baby has Down syndrome (trisomy 21) and trisomy 18 (and possibly trisomy 13 depending on the laboratory your doctor uses). If results are show an elevated risk, you would have the option of further testing to give you more definitive answers. If your results are normal, it reduces the chance for these conditions, but does not rule it out. FTS is done between 11-14 weeks gestation, and you usually get results in less than a week. FTS involves a blood draw on mom and an ultrasound.
First Trimester Screening (FTS) is a prenatal screening test performed on mom between 11-14 weeks gestation that estimates the chance of age-related chromosome differences, including Down syndrome (trisomy 21) and trisomy 18. Some labs also assess chances of trisomy 13. The test involves two steps: 1) drawing mom’s blood to measure two proteins produced during pregnancy: pregnancy–associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG), and 2) doing an ultrasound to measure the nuchal translucency (a fluid filled space on the back of the baby’s neck). Results from FTS are not yes or no, and most people who have an “abnormal” or “high risk” FTS result have typically developing, healthy babies. This test is designed to identify pregnancies that are at increased chance of these conditions and offer them further, more definitive testing. People choose to have screening tests such as FTS for various reasons, but it is important to remember that this testing is voluntary and is not required.
HOW IT WORKS
First trimester screening has two components; ultrasound to measure the nuchal translucency of the baby, and a blood draw to measure two proteins in mom’s blood.
FTS ultrasound can be performed by various providers (perinatologist, OB, radiologist, sonography technician). However, they need to have a specific certification to be able to take measurements of the nuchal translucency (NT). The NT is the fluid-filled space in the back of the baby’s neck. There should be some fluid in this area, but if there is extra fluid in the back of the baby’s neck this measurement will be wider or bigger than usual. Extra fluid, and hence a wider NT, increases the chance for Down syndrome (trisomy 21), trisomy 18, and other birth defects.
Besides taking the NT measurement, your doctor will also determine the dating of your pregnancy by measuring the fetus. Since the fetus is still very small, it is too early to be able to see very much detail. Some doctors may be able to give you a pretty good guess regarding the gender at the time of your FTS ultrasound, but in many cases gender cannot be determined by ultrasound until at least 16 weeks.
The FTS blood test looks at proteins produced naturally during pregnancy by the placenta that cross into mom’s blood stream. Research has shown that babies who have Down syndrome or trisomy 18 may have levels of these proteins that are higher or lower than usual. FTS measures these proteins and looks for certain patterns of higher or lower levels. Then a risk figure is calculated using a computer algorithm which takes into account: mom’s age, the gestational age of the pregnancy based upon the length of the fetus, number of fetuses, mom’s weight, race, whether mom smokes, and whether mom has diabetes.
The proteins measured in first trimester screening include:
- Pregnancy-associated plasma protein-A (PAPP-A): PAPP-A is produced by the placenta and crosses over into mom’s blood stream. Lower levels of PAPP-A increase the chances of Down syndrome or trisomy 18, and can also be associated with poor pregnancy outcomes, (miscarriage, low birth weight, and high blood pressure). However, keep in mind that most pregnancies that have low PAPP-A levels go on to produce healthy babies.
- Human chorionic gonadotropin (hCG): hCG is produced by the placenta and crosses over into mom’s blood stream. hCG tends to be higher in pregnancies with Down syndrome and lower in pregnancies with trisomy 18. Various forms of hCG are measured for FTS depending on the laboratory that is used (eg. free beta hCG vs. total hCG).
Your doctor will be able to tell you the day of the ultrasound more about your NT measurement. If it is abnormal (wider), he or she may offer you further testing such as chorionic villus sampling (CVS) or cell-free DNA (cfDNA) at that time. If the NT measurement is abnormal, there is a greater chance that the baby does have a chromosome abnormality, such as Down syndrome, trisomy 18, or a structural problem like a heart defect.
You should get FTS results within a week of having your ultrasound and blood draw. Results are typically called out over the phone. As talked about earlier, FTS is a screening test and never gives yes or no answers; results are typically given as a risk number (1 in 100, or 1 in 1,000). Each lab determines their cutoff for what they call a ‘high-risk’ result, but the typical cutoff for Down syndrome is 1 in 220, meaning if FTS says the risk for Down syndrome in your pregnancy is higher than 1 in 220, the result will be flagged as ‘high-risk’
Thinking about numbers like this can be difficult for people to understand, particularly if they are overloaded with information, so here is an example: let’s say you recently had FTS and you get a phone call from your doctor’s office saying ‘Your results came back and they showed a high risk for Down syndrome.’ This information often catches people off guard, but it is important to listen to what the exact risk number is. A chance of 1 in 150 and a chance of 1 in 10 are both considered ‘high-risk’ results, but most people would consider 10% to be more concerning than less than 1%.
People look at these risk figures differently. It may be helpful to look at these numbers in different ways. For example, if the doctor says the chance for Down syndrome is 1 in 10, that is the same as 10%. That means there is a 9 in 10, or 90% chance, that the baby does not have Down syndrome. Sometimes putting these numbers in a different context can help to make more sense out of them.
ABNORMAL RESULTS: My FTS is abnormal or high risk, now what?
People choose to move forward with prenatal testing for various reasons, from wanting to find out ahead of time to be prepared to wanting to find out because it may change how they move forward with the pregnancy. It is important to again mention that most pregnancies who have high-risk FTS go on to produce healthy babies. But, high-risk results can make people worried or nervous; the uncertainty of hearing that something ‘might’ be going on with your baby can produce considerable anxiety.
So, what happens next? Many doctors will refer patients with an abnormal FTS to a high-risk pregnancy doctor (perinatologist), or back to the doctor who ordered the FTS to discuss further testing options. You may also be referred to see a genetic counselor, a trained medical professional, to talk more about the test results, answer your questions, and other options for more information.
There are many options for more testing, including chorionic villus sampling (CVS), amniocentesis, cell-free DNA (cfDNA), and level II ultrasound. An important thing to keep in mind about all of these tests is that they are optional, not mandatory.
NORMAL RESULTS: What’s next?
If someone has ‘normal’ FTS results, that means that the chance for any of the conditions that the test looked at were reduced, but not eliminated. Only about 80% of pregnancies that have Down syndrome and trisomy 18 will have a positive FTS, which means 20% will have low-risk FTS results. The false positive rate, which is how often a healthy pregnancy will have a high-risk FTS test result, is around 5%.
HOW TO DECIDE
Should I have FTS?
People who are considering having FTS may find it helpful to consider the following questions:
- How would you feel if FTS results showed a high risk for a problem?
- Would you consider amniocentesis?
- Do you think this information would help you feel more prepared?
- If not, would you be ok waiting until the baby is born to know for sure if the condition is present if your FTS result returns high risk?
- For example, would you consider doing anything differently if you knew the baby had a genetic condition (e.g. prepare, consider placing baby for adoption, consider not continuing the pregnancy)?
- Does more information with the possibility of uncertainty make you anxious?
- Some women may feel comfortable with their chance of a genetic condition or are confident that, even if the baby did have a genetic condition, it wouldn’t alter their pregnancy plans. Others may prefer not to face the decision of whether or not to have an amniocentesis if the FTS comes back high risk. In these cases some women may decide not to undergo any prenatal screening.
Questions? Get in touch
Labs vary, but most will give information and risk figures for Down syndrome. Some, but not all labs may provide a risk figure for Trisomy 18. Some labs will offer a pseudo-risk for the pregnancy (not an individual risk for each baby), others will separate the risk figures out and give a risk for Twin A and for Twin B. Ask your doctor or genetic counselor if you have further questions about twin pregnancy and FTS.
During the ultrasound portion of FTS, the doctor will also try to see the nasal bone. If the nasal bone is absent, it does not mean that the baby has Down syndrome. However, it does increase the chance of Down syndrome (trisomy 21). Some laboratories can use this measurement when they calculate the overall risk. Not all doctors choose to use this measurement as a routine part of FTS. According to the American College of Obstetrics and Gynecologists, the value of the nasal bone measurement as part of screening in the general population is controversial. An individual’s ethnicity can also affect this measurement; people with Asian ancestry are more likely to have an absent nasal bone on a first trimester ultrasound.
An increased NT measurement does increase the chances of the baby having a chromosome abnormality, such as Down syndrome. However, increased NT also has been associated with heart defects, other genetic disorders, and other birth defects, and can increase the chances of miscarriage. Again, if your baby’s NT measurement is bigger than usual (abnormal), your baby may be healthy and well, but your doctor may offer you further testing to determine if there is anything going on with the baby that could be causing the bigger NT. Further testing offered may include a level II ultrasound in the second trimester and/or a fetal echocardiogram (detailed look at the baby’s heart through ultrasound in the 2nd trimester).
FTS is designed to cast a wide net using the ultrasound and blood test to try to determine which pregnancies could be at an increased risk for a chromosome problem. In general, approximately 5% (1/20) of all women who have FTS will have an abnormal or positive result. In reality, most people that have an abnormal result end up having babies that do not any health problems. This does not mean the test is “wrong”; it is just the way it works. This is a screening test, not a diagnostic test. In order to know with more certainty if the baby actually has the condition, further testing is required. Diagnostic testing, such as CVS or amniocentesis, is the only way to give you ‘yes’ or ‘no’ answers.