What Is Prenatal Genetic Screening?
Prenatal genetic screening: explained
During pregnancy, you’ll encounter many decisions about your prenatal care, including options for prenatal genetic testing and screening. When you’re considering these options, it’s important to understand what genetic screening means, what tests may be available to you, and what genetic screening can and cannot tell you about the health of your baby.
What is the difference between screening vs. diagnostic testing?
Screening tests are typically performed in healthy people as part of a routine assessment for health conditions. Screening is meant to identify those who might have an increased chance of a disease or diagnosis. Importantly, though, screening is not able to definitively diagnose a condition – nor is screening able to definitively rule out a condition. Examples of screening include mammograms (for breast cancer risk) and pap smears (for cervical cancer risk). Mammograms and pap smears aren’t typically done because your doctor suspects you have breast cancer or cervical cancer, respectively. Instead, these screens are done to help identify those with an increased chance of disease. Once these high-risk individuals are identified, follow-up testing can be done, and an early diagnosis can be made if the disease is present.
Because screening tests are not able to diagnose a condition, follow-up testing is typically recommended if screening reveals an increased chance or high risk. For example, if a pap smear finds abnormal or atypical cervical cells, the next step often includes a biopsy to remove a small piece of cervical tissue. Then, the suspicious cells can be examined in a lab to confirm or rule out a cervical cancer diagnosis. In contrast to a screening test, diagnostic tests are able to give a “yes” or “no” answer regarding a health condition – rather than offering a risk or probability.
What is prenatal genetic screening? What prenatal genetic screens are available?
Prenatal genetic screening refers specifically to screening tests that assess the chance for genetic conditions during pregnancy. Prenatal genetic screens often test for chromosome conditions such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13. Multiple types of genetic screening are available during pregnancy. These screens vary in how and when they are completed, what they test for, and how well they perform. Read on to learn about prenatal screening options to see which type of screening test may be right for you.
- Ultrasound: Dating back to the early 1970s, fetal ultrasounds have been a widely used tool to screen for birth defects and physical differences during pregnancy.
The nuchal translucency (NT) ultrasound is one common tool used to identify anatomical clues for fetal chromosome conditions in the first trimester. During an NT ultrasound, the sonographer measures the size of the fluid-filled space at the back of the baby’s neck – this is the nuchal translucency. More fluid at the back of the neck indicates an increased risk for chromosome conditions like Down syndrome (trisomy 21) or Turner syndrome (monosomy X). An increased NT measurement can also be a sign that a physical difference, such as a heart defect, may be present.
In the second trimester, an anatomy ultrasound scan is typically performed to look at the baby’s physical development from head to toe. Certain findings on an anatomy scan may be a sign of increased risk for chromosomal or genetic conditions in the pregnancy. For example, a bright spot on the baby’s bowel, known as echogenic bowel, can indicate an increased risk for Down syndrome or cystic fibrosis. Because ultrasounds are a screening tool, not a diagnostic test, they cannot definitively determine whether a baby has a chromosomal or genetic condition. Still, the results of ultrasounds like an NT scan or anatomy scan can help inform families and may be useful as they decide whether to pursue additional prenatal genetic testing.
- Maternal Serum Screening: Maternal serum screening is a blood-based prenatal screen that looks for chromosomal or genetic conditions and certain birth defects. Maternal serum screening works by measuring the levels of specific analytes (proteins and hormones) present in your blood during pregnancy. Pregnancies with chromosome variations, such as Down syndrome, typically have higher or lower levels of these analytes compared to pregnancies that do not have a chromosome condition. For example, alpha-fetoprotein (AFP) is one protein measured by maternal serum screening. AFP levels are typically lower in women carrying a baby with Down syndrome compared to AFP levels in those whose babies who do not have Down syndrome. By analyzing the pattern of analyte levels in a pregnant person’s blood, a risk assessment can be made for specific conditions. Depending on the screen performed, these conditions might include chromosome conditions like Down syndrome and trisomy 18, as well as physical differences like neural tube defects. Because maternal serum screening is a blood screening test, it is non-invasive and does not pose any risks to you or your baby.
- Non-invasive prenatal screening (NIPS): Noninvasive prenatal screening (NIPS), also known as noninvasive prenatal testing (NIPT), is a screening test available early in pregnancy, no matter your age. NIPS can be performed on a simple blood sample and works by analyzing the small fragments of DNA circulating in your blood. These fragments are called cell-free DNA (cfDNA) because the DNA is free-floating in blood – unlike most DNA found inside the nucleus of a cell. During pregnancy, your bloodstream contains a mix of cfDNA from your own cells and cfDNA from the placenta. Since your placenta and your baby originated from the same group of cells (embryo), placental DNA is usually identical to fetal DNA. Therefore, analyzing placental cfDNA in your bloodstream makes it possible to screen for chromosomal conditions in your baby. If NIPS detects an increased amount of DNA coming from a certain chromosome, this indicates an increased chance for a trisomy (extra copy of that chromosome). For example, if NIPS detects a higher amount of chromosome 18 cfDNA than expected, this would result in a screen positive or “high risk” result for trisomy 18. NIPS can be done as early as 9 weeks into pregnancy, and because it is non-invasive, it is safe for both you and your baby.
- Genetic carrier screening: In contrast to NIPS and maternal serum screening, which both screen for chromosomal conditions in the baby, genetic carrier screening looks at the DNA (genetic material) of parents or couples to see whether they carry recessive genetic conditions that could be passed on to a baby. Humans typically have two copies of every gene - one copy from their genetic mother, one copy from their genetic father. Recessive inheritance means that a person needs to inherit two non-working copies of a gene, one from each genetic parent, in order to have the recessive condition. Examples of recessive genetic conditions include cystic fibrosis, sickle cell anemia, thalassemia, and spinal muscular atrophy. If one genetic parent is a carrier for a recessive condition, but the other parent is not a carrier, it is unlikely that their baby would have that condition. If both genetic parents are carriers for the same recessive condition, there is a 1 in 4 chance (25%) for a baby to be affected. If two partners are identified as carriers of the same recessive condition, they can consider prenatal diagnostic genetic testing to see whether their baby is affected by the condition.
- Authored by Kelly Miller, MS, LCGC | Genetic Counselor
Our next blog "Genetic Screening: Accuracy and Results" provide more insights into the accuracy and the various results.
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