Prenatal Genetic Screening Tests: A Pharmacist's Guide

Pharmacists can play an important role in pregnancy health education. Prenatal vitamins contain folic acid, which can prevent spina bifida and other neural tube defects, which is a great counseling point that pharmacists can provide. Prenatal genetic screening tests have become very common over the past several years to screen for birth defects. As accessible health care professionals, pharmacists may receive questions regarding the different tests. Pharmacist collaboration with an OB-GYN would present a unique practice setting opportunity for pregnancy health counseling. This article will review the different prenatal genetic screening tests.

What prenatal genetic screening tests are available?

Screenings utilize blood tests and ultrasound exams to identify the risk of a fetus having aneuploidy, or an abnormal number of chromosomes.¹ It is important to remember that these are screening tests and are not diagnostic. The most common type of aneuploidy is Down syndrome (trisomy 21). The screening tests can also identify the risk of neural tube defects and abdominal, heart, and facial defects.

There are four prenatal genetic screening tests available:¹

• First-trimester screening includes a blood test and ultrasound exam performed between 10 and 13 weeks of pregnancy. The ultrasound exam, called the nuchal translucency screening, measures the thickness of a space at the back of the fetus’s neck. An abnormal measurement means there is an increased risk of Down syndrome and has also been linked to heart, abdominal wall, and skeletal defects.
• Second-trimester screening includes the 'quad' blood test that screens for Down syndrome, trisomy 18, and neural tube defects, and is conducted between 15 and 22 weeks of pregnancy. The ultrasound is performed between 18 and 20 weeks of pregnancy and checks for physical defects of the brain, spine, facial features, abdomen, heart, and limbs.
• The combined first and second trimester screening are more accurate than a single test, but results are generally not available until the second trimester.
• Cell-free DNA testing, also known as noninvasive prenatal testing (NIPT), analyzes the small amount of DNA that is released from the placenta into a pregnant woman’s bloodstream. The blood test screens for Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities. The NIPT can be conducted at 10 weeks of pregnancy, and results are available within approximately 1 week.

How accurate are the screening tests?

The quad screen is approximately 80% accurate in identifying Down syndrome.² However, there is about a 5% risk of a false-positive result.² Patients should also keep in mind that the following factors can affect the screening: miscalculation of the baby’s age, maternal weight, multiple pregnancy, diabetes, and in vitro fertilization.² Studies have demonstrated that the NIPT is about 97-99% accurate at identifying the risk of Down syndrome in a fetus with a false-positive risk of less than 1%.^3,4 The use of the NIPT has become popular due to its high level of accuracy. However, it may not be covered under insurance for all patients, and the cost varies widely. The test is generally covered through insurance for the following patients: those aged 35 years or older at the time of delivery; those that have screening tests with abnormal results; and those with a history of aneuploidy. Physicians generally recommend prenatal diagnostic tests, such as amniocentesis or chorionic villus sampling, for patients with positive NIPT results.

Do these screenings increase anxiety levels among patients?

There is an increased move for screening all pregnant patients, even those that are at low risk. One study evaluated women undergoing prenatal genetic testing and found that those with high risk combined first trimester results have high levels of anxiety.⁵ These screenings can be valuable to prevent unnecessary invasive diagnostic tests. However, waiting for the results can be stressful for patients. More pregnant patients are moving towards the NIPT screening as the results are available quickly, and it has a higher level of accuracy.

References

• The American College of Obstetricians and Gynecologists. Prenatal genetic screening tests. ACOG website. www.acog.org/Patients/FAQs/Prenatal-Genetic-Screening-Tests. Accessed April 3, 2018.
• Mayo Clinic. Quad screening. Mayo Clinic website. www.mayoclinic.org/tests-procedures/quad-screen/about/pac-20394911. Accessed April 4, 2018.
• Taylor-Phillips S, Freeman K, Geppert J, et al. Accuracy of non-invasive prenatal testing using cell-free DNA for detection of Down, Edwards, and Patau syndromes: A systematic review and meta-analysis. BMJ Open. 2016;6(1):e010002. doi: 10.1136/bmjopen-2015-010002.
• Bianchi DW, Parker RL, Wentworth J, et al. DNA sequencing versus standard prenatal aneuploidy screening. N Engl J Med. 2014;370:799-808.
• Richmond Z, Fleischer R, Chopra M, et al. The impact of non-invasive prenatal testing on anxiety in women considered at high or low risk for aneuploidy after combined first trimester screening. Prenat Diagn. 2017;37(10):975-982.