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Prenatal Screening Tests

December 30, 2020

Pregnancy is miraculous, filled with joy and wonder. But it can also be an extremely nerve-racking time. Even with all of our modern technology and scientific advancement, during pregnancy both doctors and prospective parents often don’t know for sure whether the baby will suffer from a genetic abnormality or a neural tube defect. 


This is where screening tests come into play. Screening tests are tests which can help to predict whether the baby will have a genetic disorder or a neural tube defect. They include nuchal translucency screening, maternal serum screening, non-invasive prenatal testing, and the more invasive CVS and amniocentesis.


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Pregnancy and Prenatal Tests

Pregnancy and Prenatal Tests

What do these tests find?

A nuchal translucency screening (NT) is performed from week 11 until week 13 of pregnancy. It is a specialized ultrasound that measures the thickness of the nuchal fold (found at the base of the baby’s neck). The presence of increased fluid in the nuchal fold points to a higher likelihood that the baby might have a genetic disorder. However (and this is very important), because it’s a prenatal screening, it can only tell you how likely it is (statistically) that one’s baby has a chromosomal disorder. It cannot provide any definitive diagnosis or predict with 100% accuracy that the baby has a genetic abnormality.


Doctors often perform both an NT and accompanying blood tests. They do this because NT results by themselves have a relatively high margin of error. In fact, the test will show up as normal for as many as 25 to 30 percent of babies with Down syndrome or other genetic disorders. 


Maternal serum screening is a blood test available to pregnant women who want to know about their chance of having a baby with a chromosome disorder. This test may be performed either at around 10 weeks (combined first-trimester screening test) or between 14 and 20 weeks (second-trimester maternal serum screening test). 


The goal of maternal serum screening tests is not to diagnose health conditions but to identify women who should be offered further testing to detect a genetic condition. Therefore, if the baby is identified as having an “increased chance” result, the mother will probably be offered a diagnostic test. Most of the time though, babies who have an “increased chance” will not be born with Down syndrome, Edwards syndrome, or a neural tube defect.


If doctors use the combined results of both the first trimester screening and the NT screening, they will know with up to 92% accuracy whether the baby will have Down syndrome.


But 92% is not 100%—a more recent and more reliable screening test, the NIPT (or non-invasive prenatal testing), will detect nearly all pregnancies affected by Down syndrome, Edwards syndrome, and/or Patau syndrome. It also poses no risk to the fetus because all that is required is drawing blood from the pregnant woman.


Risks and decisions

Although NIPT primarily tries to detect chromosomal disorders caused by an extra or missing copy of a chromosome that characterizes a condition like Down syndrome, it may also include screening for additional chromosomal disorders caused by missing or copied sections of a chromosome. 


While the NIPT is more reliable than other screening tests, it is not foolproof. Sometimes, it results in a false positive or a false negative.  A false positive occurs when the test results indicate an increased risk of a genetic disorder, but the fetus does not actually have the disorder. A false negative occurs when the fetus has the disorder but the results show that it has a decreased risk. 


Chorionic villus sampling (CVS) and amniocentesis (amnio) are invasive diagnostic tests performed when there is an increased risk that the baby may have genetic disorders or birth defects. This includes if the pregnant woman will be 35 or older on the baby’s due date, if the couple already has a baby with a chromosomal or neural tube disorder, or if either of the parents are known to be carriers of a genetic disorder.


CVS can be performed early on in the pregnancy, but it won’t detect defects such as neural tube defects, birth defects, or Rh incompatibility, which an amnio will detect. Undergoing either CVS or an amniocentesis carries with it a risk of causing a miscarriage, leaking of amniotic fluid, vaginal bleeding, and harm to the baby, so it’s a decision that shouldn’t be taken lightly.


Although screening for chromosomal abnormalities should be offered to all pregnant women, ultimately the decision to be tested lies with the pregnant woman and remains highly personal.







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