Fetal Screening & Diagnostic Procedures
Cell-Free Fetal DNA
Cell Free Fetal DNA- also called- Noninvasive prenatal testing (NIPT), is a method of determining the risk that the fetus will be born with certain genetic abnormalities. This testing analyzes small fragments of DNA that are circulating in a pregnant woman’s blood. These small fragments arise when cells die off and get broken down and their contents, including DNA, are released into the bloodstream.
During pregnancy, the mother’s bloodstream contains a mix of cfDNA that comes from her cells and cells from the placenta. These cells are shed into the mother’s bloodstream throughout pregnancy. The DNA in placental cells is usually identical to the DNA of the fetus. Analyzing cfDNA from the placenta provides an opportunity for early detection of certain genetic abnormalities without harming the fetus.
NIPT is most often used to look for chromosomal disorders that are caused by the presence of an extra or missing copy (aneuploidy) of a chromosome. NIPT primarily looks for Down syndrome (trisomy 21), trisomy 18, trisomy 13, and extra or missing copies of the X chromosome and Y chromosome (the sex chromosomes). Other rare conditions may be investigated as well depending on the indication. The accuracy of the test varies by disorder. NIPT is beginning to be used to test for genetic disorders that are caused by changes (variants) in single genes. As technology improves and the cost of genetic testing decreases, researchers expect that NIPT will become available for many more genetic conditions. Nevertheless, there are important limitations to this test that patients need to be aware of.
NIPT is considered noninvasive because it requires drawing blood only from the pregnant woman and does not pose any risk to the fetus. NIPT is a screening test, which means that it will not give a definitive answer about whether or not a fetus has a genetic condition. The test can only estimate whether the risk of having certain conditions is increased or decreased. In some cases, NIPT results indicate an increased risk for a genetic abnormality when the fetus is actually unaffected (false positive), or the results indicate a decreased risk for a genetic abnormality when the fetus is actually affected (false negative). Because NIPT analyzes both fetal and maternal cfDNA, the test may detect a genetic condition in the mother.
There must be enough fetal cfDNA in the mother’s bloodstream to be able to identify fetal chromosome abnormalities. The proportion of cfDNA in maternal blood that comes from the placenta is known as the fetal fraction. Generally, the fetal fraction must be above 4 percent, which typically occurs around the tenth week of pregnancy. Low fetal fractions can lead to an inability to perform the test or a false negative result. Reasons for low fetal fractions include testing too early in the pregnancy, sampling errors, maternal obesity, and fetal abnormality.
For additional information, or to discuss whether this test is appropriate for you, please ask our providers!
Nuchal Translucency Screening
Nuchal translucency screening is a test to help determine if there is an increased risk of certain chromosome and structural disorders with your unborn fetus. The full test includes both a blood sample from the mother and an ultrasound to measure the "nuchal translucency"- a fluid filled area between the muscles and skin on the back of the fetus' neck- as well as to look at the early structural development of the fetus. Other ultrasound markers that can be incorporated to increase the detection rate of the test include the presence of the nasal bone, and evaluation of flow through both the fetal tricuspid valve and ductus venosus.
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The blood test component can be done as early as 9 weeks and measures the levels of PAPP-A (pregnancy associated plasma protein A), AFP and free beta HCG in the maternal blood. These results are then coupled with the measurement of the nuchal translucency and other ultrasound markers, performed between 11 and 14 weeks, to arrive at a final numerical risk for this fetus.
The test is helpful to determine if there is an increased risk of trisomy 21 (Down syndrome), trisomy 13 or 18 in your unborn fetus. The test is not a diagnostic test- it only helps us know if the risk is higher or lower than we would expect based on your age- but can nonetheless provide significant reassurance to parents when the result is normal. For greater accuracy, a second trimester blood specimen may be incorporated into the first trimester test if desired; this is called a "Sequential Screen".
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The size of the nuchal translucency measurement is also helpful to determine if there is an increased risk of several other types of birth defects. For example, the measurement may be increased in cases where there is a higher risk for a heart defect or other problem in the baby.
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Because the blood test component is measuring "analytes" or hormones that originate in the placenta, it can also help us evaluate placental function and determine if your pregnancy should be followed more closely to watch issues like fetal growth. This is particularly true when both a 1st and 2nd trimester blood specimens are submitted.
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Other optional components of the test include the evaluation of the presence of absence of the "Y" chromosome (for fetal sex determination), and the option of early assessment for preeclampsia risk. In order to determine if there is an increased risk of preeclampsia- a potentially severe complication of pregnancy- an additional placental analyte called PIGF is measured.
For preeclampsia risk assessment, the blood specimen must be paired with a measurement of blood pressure in the pregnant woman, and an ultrasound study (Doppler) looking at flow and resistance through the arteries that supply the uterus. Once these parameters are obtained, your individual risk calculation for preeclampsia in this specific pregnancy can be obtained. Many women with a history of severe preeclampsia or other risk factors find this test extremely helpful.
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Uterine artery Doppler measurements, as well as all of the 1st trimester fetal markers for chromosome disorders, require certification or 'licensing' through a credentialing body such as the Fetal Medicine Foundation. Dr Solomon is the only physician in Arizona certified in all ultrasound markers as well as uterine artery/preeclampsia screening.
Chorionic Villus Sampling
Chorionic villus sampling (CVS) is a prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The sample can be taken through the cervix (transcervical) or the abdominal wall (transabdominal). The chorionic villi are part of the placental tissue and in 99+% of instances, share the same genetic makeup as the developing fetus. By obtaining a sample of the villi or placenta, we are effectively obtaining a sample of the fetus' DNA, which allows for definitive diagnostic testing for a variety of conditions. In addition to common disorders like Down syndrome, the DNA obtained can also be used to test for other genetic disorders such as cystic fibrosis, sickle cell disease, and many other disorders as appropriate to your case.
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The test can be performed as early as 10 weeks, so is ideal for arriving at a certain diagnosis early in the pregnancy. This is helpful for patients who may have a personal or family history of a genetic disorder, an abnormal screen for chromosome disorders, are of advanced maternal age, or where there is an early structural problem identified in the fetus by ultrasound. The sample obtained is then sent to the appropriate genetics lab to perform the desired testing.
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As with all diagnostic procedures, there is a very slight risk of miscarriage associated with the test. This rate is markedly reduced when the person performing it has extensive experience in fetal testing and diagnostic procedures. This risk is estimated at less than 1% overall, but depends on the exact reason the test is performed. For example, if there is a significant concern of a genetic disorder in the fetus before the test, the likelihood of miscarriage is elevated even before the CVS is performed.
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Please ask our providers if you have any additional questions or to find out if this test may be appropriate for you. A video below shows a trans abdominal CVS performed by Dr Solomon. Please note that the fetus is always visualized for safety purposes.
Amniocentesis
Amniocentesis or "puncture of the amnion," refers to the process of aspirating a small amount of the amniotic fluid that surrounds the fetus in the mother’s womb. The amnion is a thin-walled sac of fluid in which a developing fetus is suspended during pregnancy. The function of the fluid is to protect the unborn baby while it is in its developmental stages, and to allow development of the organs. It is largely a product of fetal urination. As a result, the amniotic fluid contains the baby’s cells and DNA, and testing for a variety of conditions can be performed as described in the section above (CVS).
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During the sampling procedure, a very fine needle is inserted through the mother's abdomen into the uterus and amniotic sac, and approximately 20cc of amniotic fluid is obtained for testing. Ultrasound is used to guide needle placement and collect the sample, thereby minimizing the risk of fetal injury or the need for repeated needle insertions. The amount of fluid withdrawn is negligible in comparison to the total volume, and is replaced by the fetus in less than 12h. This relatively painless procedure is performed in the office. As with CVS, there is a slight risk to the procedure, but this is estimated at less than ¼ of a percent and is minimized in the hands of extremely experienced physicians.
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Because amniotic fluid also contains various hormones made by the fetus, the test can also be used to clarify a possible diagnosis of spina bifida or suspected fetal infection, as bacteria or viruses would then be found in the fluid. To find out if this procedure is appropriate for you, please discuss further with our providers.
Other Fetal Procedures
Other diagnostic and fetal procedures are available and indicated in the setting of very specific and uncommon pregnancy complications. In some cases, specific fetal treatment centers are needed to deal with the complexity of these cases. Please see the page on on this website for additional details. However, certain less complex procedures can be performed locally either by Dr Solomon or as part of a team approach with other Phoenix area specialists. These include:
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- amnioreduction (removal of excess amniotic fluid)
- amnioinfusion (placement of fluid into the uterus for visualization and treatment)
- placental biopsy
- fetal bladder drainage
- fetal cyst or mass drainage
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