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Prenatal screening tests analyze the risk of a developing fetus having a chromosomal disorder or a birth defect — either inherited from the parents, or due to a random anomaly. Both noninvasive prenatal testing, often called NIPT, and maternal serum screening are used to analyze such risks, but they’re quite different in how they reach their results.

What is Noninvasive Prenatal Testing?

NIPT is a screening test used to determine whether the fetus has a high risk of having a chromosomal disorder. NIPT is mainly used to analyze the risk of trisomy disorders — chromosomal disorders that develop when a third copy of a chromosome is inherited, like Down syndrome — but it can also be used to calculate the risk of sex chromosome disorders.

NIPT is a noninvasive test conducted on a sample of the mother’s blood. Circulating in the mother’s blood is DNA from the placenta, called cell-free fetal DNA, that is identical to the fetus’ DNA. At gestation week 10, about 10 percent of the cell-free DNA in a woman’s bloodstream belongs to the fetus. Laboratory technicians analyze the cell-free fetal DNA for abnormalities in the number of chromosomes to determine the risk that a condition exists.

NIPT is routinely offered to women who:

— Are 35 years or older

— Have had a previous pregnancy with Down Syndrome or another chromosomal disorder

— Received abnormal results from an ultrasound that showed a possible problem with the fetus

However, more obstetricians are offering this test to all pregnant women. NIPT can be performed as early as week 10 in pregnancy and is touted for its accuracy — between 97.4 and 99.3 percent for the most common trisomy disorders (Down syndrome, Edward’s syndrome, and Patau syndrome).

How is Maternal Serum Screening different from NIPT?

Maternal serum screening, also called “triple screen” or “quad screen”, is a noninvasive screening test that measures specific substances in the mother’s blood. This test is most commonly used to detect Down syndrome (trisomy 21), Edward’s syndrome (trisomy 18), and  open neural tube defects like spina bifida.

The substances measured in serum screening include:

— Alpha-fetoprotein (AFP)

— Human chorionic gonadotropin (hCG)

— Unconjugated estriol (uE3)

— Inhibin A (included in quad screening)

The amount of each of these substances varies depending on the week of gestation. When the sample is analyzed in the laboratory, these substances are compared against the average amounts normally seen in women for the specific gestation week. For example, when a fetus has an extra copy of the twenty-first chromosome (Down syndrome), AFP and uE3 levels tend to be lower than average, while hCG and DIA tend to be higher than average.

Maternal serum screening can be conducted in the first trimester or the second trimester but is not considered to be as accurate as NIPT.

Which Test is Right for You?

For both NIPT and maternal serum screening, if these tests indicate that there is a high risk that the child has a chromosomal abnormality or a physical deformity, a diagnostic test will need to be conducted. Diagnostic testing will tell your doctor whether the fetus does, in fact, have the suspected condition.

If you’re interested in prenatal screening, consider speaking to a genetic counselor to further discuss the differences between NIPT and maternal serum screening. A genetic counselor will help you understand the benefits to each test as well as what the results mean for you and your child.


Resources:

https://www.parents.com/pregnancy/stages/1st-trimester-tests/nipt-a-crash-course-in-non-invasive-prenatal-testing/

https://www.vcgs.org.au/tests/maternal-serum-screening

https://genetics.emory.edu/documents/resources/Emory_Human_Genetics_About_Maternal_Serum_Screening.pdf

https://www.nature.com/articles/gim200477

https://www.mayoclinic.org/diseases-conditions/spina-bifida/symptoms-causes/syc-20377860

https://www.integratedgenetics.com/patients/pregnancy/serum-screening

https://obgyn.onlinelibrary.wiley.com/doi/full/10.1002/pd.4119

https://www.ncbi.nlm.nih.gov/pubmed/26781507

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Category: Pregnancy

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