First trimester screening (FTS)
To the Point- First trimester screening (FTS) can tell you if your baby’s chances of having Down syndrome (Trisomy 21), Trisomy 18 (and possibly Trisomy 13 depending on the laboratory your doctor uses) are increased. If results are abnormal, you need further testing to give you more definitive answers. If your results are normal, it does not rule out these conditions. FTS is typically 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.
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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 abnormalities, including Down syndrome (Trisomy 21) and Trisomy 18 and some labs also assess chances of Trisomy 13. The test involves 2 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 (which is a fluid filled space on the back of the baby’s neck). Results from FTS are not yes or no. Most people who have an “abnormal” or “high risk” FTS result have typically developing, healthy babies. This test is designed to identify individuals whose babies 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, however, this is not a mandatory test and it is up to you!
HOW IT WORKS
First trimester screening has 2 components; ultrasound to measure the nuchal translucency of the baby and 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 be certified as the nuchal translucency (NT) measurement requires specific measurements. Nuchal translucency is a term that describes the fluid filled space in the back of the baby’s neck. If there is extra or more fluid than normal (some fluid is normal) 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 (gestational age) by measuring the fetus. Since the fetus is still very small, it is too early to get a good look at the other physical structures. Some doctors may be able to give you a pretty good “guess” regarding the gender at the time of your FTS ultrasound, but many will qualify the guess with telling you not to go home and paint the nursery blue or pink just yet.
FTS Blood draw
There are proteins produced naturally during pregnancy by the placenta that cross into mom’s blood stream. The FTS measures the levels of these proteins in mom’s blood. 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. So, FTS measures these proteins and looks for certain “patterns” of higher or lower levels and then using a computer algorithm which includes your age, gestational age of the pregnancy based upon the length of the fetus, number of fetuses, your weight, race, smoking status and diabetic status, a risk figure is calculated for each of these conditions.
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. Lower levels of PAPP-A can also be associated with poor pregnancy outcomes, such as, miscarriage, low birth weight and high blood pressure during the pregnancy (gestational hypertension). However, keep in mind that healthy babies and pregnancies can have low PAPP-A levels.
Human chorionic gonadotropin (hCG)- various forms of hCG are measured for FTS depending on the laboratory that is used (eg. free beta hCG vs. total hCG). hCG is produced by the placenta and crosses over into mom’s blood stream. hCG tends to be higher in babies with Down syndrome and lower in babies with Trisomy 18.
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 noninvasive prenatal testing (NIPT) 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 abnormality such as a heart defect.
You should get FTS results within a few days 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. For FTS, results are typically given as a risk figure and based on how high the risk is, the lab will indicate if it is abnormal/high risk or normal/not high risk. Each lab may vary a bit, but a typical cut-off for high risk for Down syndrome in the first trimester is 1/220, or <0.5%. In other words, if your risk figure is higher than 1/220 for Down syndrome, the report would say that your results are abnormal for Down syndrome.
Let’s give an example. Let’s say you had FTS and you got a phone call from your OB or perinatologist/Maternal-Fetal Medicine office saying, “your results came back indicating a elevated chance or high risk for Down syndrome.” This may not be what you were expecting and often times it is hard to concentrate and listen to what is said beyond that statement, but hopefully the office will also tell you what the actual chances are for Down syndrome (if not, you should ask them). In this case, let’s just say it is 1/200 (or 0.5%). What this really means is that if they looked at 200 women who were the same age as you and had the same chemical levels and nuchal translucency measurement, 1 out of those 200 babies would have Down syndrome and the other 199 would not have Down syndrome. Another way of looking at it is that the chance of having a baby without Down syndrome is 99.5%. So, even though the test results are deemed “abnormal”, the odds are the baby does not have Down syndrome.
Here’s one more example that would apply to you if you are over 35 at delivery. The FTS uses maternal age in its calculation, and we have already talked about the fact that the chances of things like Down syndrome increase as mom’s age increases. What this can mean is that the chance of you having a high risk or abnormal sequential screen increases a bit as you age. For example, if you are 42, your approximate background chance for Down syndrome is 1/64. This chance alone is higher than the abnormal cut-off for the FTS (typically around 1/270 for Down syndrome). If a 42-year-old woman with an age-related background chance of Down syndrome of approximately 1/64 has a FTS result of 1/200 (0.5%) for Down syndrome, the result is abnormal. However, this is actually a lower risk than her background age-related risk of 1/64 (1.6%).
Risk figures can be confusing and how they are presented can really make a difference in your perception and understanding. For example, if your chances for Down syndrome came back 1/20, this means that there is a 95% chance that the baby does NOT have the condition. So, make sure you ask your provider or genetic counselor if you have questions about the risk figures or the results in general.
ABNORMAL RESULTS – My FTS is abnormal or high risk, now what?
People choose prenatal testing for various reasons, some to find out information to simply have a heads-up and be prepared, others because they are uncertain what decisions they would make if their baby did have an abnormality. What we know is that people hope that their baby is healthy and well. As explained above in the FTS results section, most likely your baby does not have the condition at hand. However, if you are like most people, you will be at least a bit worried. It is hard to hear that something “might” be going on with your baby.
So, what happens next? Many obstetricians will refer patients with an abnormal FTS to a high-risk pregnancy doctor (perinatologist or Maternal-Fetal Medicine specialist- hyper text) or back to the doctor who performed the FTS to discuss further testing options. Depending on your provider, you may also be referred to a genetic counselor to discuss the test results further. Genetic counselors are trained medical professionals who can explain the test results in detail, answer your questions and discuss all of the further testing options available to help you make the best decision based on your personality, beliefs and values.
Further testing options that will be discussed include diagnostic options that can give you more definitive answers such as, chorionic villus sampling (CVS) and amniocentesis. Other screening tests include noninvasive prenatal testing (NIPT) and level II ultrasound.
An important thing to keep in mind is that all further testing options, including level II ultrasound, noninvasive prenatal testing, CVS and amniocentesis are optional, not mandatory. See specific tests/prenatal (hypertext to specific tests->prenatal page) for more information on these other testing options.
NORMAL RESULTS on FTS – What’s next?
In terms of the conditions screened for on FTS, chances are decreased that your baby has one of these conditions. However, FTS does not detect all cases of Down syndrome or Trisomy 18. Approximately 80% of babies who have one of these conditions will have an abnormal or high-risk FTS, however, approximately 20% will have normal FTS results. Many people do not pursue further screening or diagnostic testing if they have a normal FTS. However, if you want more information, there are other screening tests available including, level II ultrasound, noninvasive prenatal testing (NIPT) and a diagnostic tests including CVS and amniocentesis.
Your doctor will likely offer you a second trimester blood draw to look solely at the alpha-fetoprotein (AFP) level in your blood, called MSAFP (maternal serum alpha-fetoprotein) or a comprehensive ultrasound to evaluate for fetal abnormalities including open neural tube defects (ONTDs). Elevated MSAFP increases the chance for open neural tube defects, such as, spina bifida and anencephaly, and also other birth defects involving the development of the baby’s abdominal area.
HOW TO DECIDE
Should I have FTS?
The following are some questions and thoughts to consider that may be helpful as you decide whether or not FTS is right for you…
- How would you feel if results indicated a higher risk for a genetic condition?
- 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 would prefer definitive answers and since FTS cannot provide that, they may choose to go straight to a more definitive test, such as amniocentesis.
- On the other hand, 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. Or they 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.
Ultimately, the decision to have FTS or any other prenatal screening or diagnostic test is yours to make and should reflect your own personal beliefs, values, needs and personality.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.
Another measurement that can be taken during the FTS is looking for the presence or absence of a fetal nasal bone. If the nasal bone is absent, it does not mean that the baby has Down syndrome, however, it may increase the chance of Down syndrome (Trisomy 21) or other chromosome abnormalities. Some laboratories can incorporate this measurement into the overall risk assessment calculation of FTS. However, 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. Also of note, ethnic background can play a significant role in the chances of an absent nasal bone.
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, however, 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 find people that are at a certain risk level or higher for Down syndrome or Trisomy 18 (possibly Trisomy 13, depending on lab). In general, approximately 5% (1/20) of women who have FTS will have an abnormal or positive result. But, in reality most people that have an abnormal result actually have babies that do not have the condition at hand. 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 definitive answers as to whether or not your baby has the conditions screened for in FTS.
Another thing to keep in mind with FTS is that it does not pick up all cases of Down syndrome or Trisomy 18. In general, FTS detects over 80% of babies with Down syndrome and approximately 80% of those with Trisomy 18. In other words, you could have a normal FTS and your baby could still have one of these conditions.