Understanding Prenatal Screening and Testing

Black pregnant woman talking with a doctor. The doctor is an Asian woman.

Front Matter

All resources available at the National Center for Prenatal and Postnatal Resources are offered as information resources and should not be relied upon as a substitute for medical advice from a medical care provider.

This online prenatal testing information tool provides the contents found in our “Prenatal Screening and Testing for Chromosome Conditions” pamphlet, as well as links to additional resources and more detailed information about chromosome conditions. “Prenatal Screening and Testing for Chromosome Conditions” is also available as a printed pamphlet, along with many other printed and online resources, at Lettercase.org.

This pamphlet was prepared with assistance from representatives of The National Society of Genetic Counselors with funding provided by the U.S. Administration on Intellectual and Developmental Disabilities (AIDD), though no official endorsement from AIDD should be inferred.

Overview

Pregnancy brings with it many changes, emotions, and appointments. During those visits, doctors offer some tests that are just part of maintaining a healthy pregnancy, such as glucose and iron tests. They also offer some optional tests that can provide genetic information about the pregnancy.

Then, patients can make decisions about prenatal genetic testing based on their own needs and values. They can choose whether or not they want these tests; which tests they want if they choose testing; and how they might use the genetic information.

While most babies are not born with a significant genetic condition, prenatal testing includes screening and diagnostic tests for those genetic conditions that can be found during pregnancy. However, no test available right now can find all possible conditions.1

White man reading a book to a baby boy with Down syndrome who is sitting on his lap.

Prenatal Testing Choices

The decision about whether or not to use prenatal genetic testing is very personal and based on a patient’s values and needs. When prenatal testing is offered, patients often find it helpful to discuss their thoughts and feelings about testing with their obstetric medical provider and/or genetic counselor and how they might use the results. That way they can address any areas of concern both before and after testing, as needed.

Some people might choose to have prenatal testing to arrange specific birth plans. There are some who might want prenatal test results so they can start treatments as soon as possible after birth, such as hormone therapy for sex chromosome conditions. Some might want to prepare emotionally for raising a child with a health issue or disability. Others might want to learn more about the condition or, in some cases, prepare for a baby who is not likely to survive. Some might opt not to continue a pregnancy or create an adoption plan for a child with a disability.

Some patients might not want any prenatal testing and feel it would not be helpful or that it would cause unwanted stress and worry.2

You can also find more information about prenatal screening and testing at the Genetic Support Foundation.

Young White man with Klinefelter syndrome looking at a tablet outside. He is standing next to a smiling White woman.

Screening Tests

Screening Tests

Prenatal screening includes ultrasounds and tests on the pregnant woman’s blood. These tests are usually done in the first and second trimester. Screening tests can determine if there is a higher or lower chance for some genetic conditions, such as Down syndrome, trisomy 18, and spina bifida. For example, a pregnant woman might be told that her odds for having a baby with Down syndrome are 1 in 500. This number depends on her individual test results, and a medical professional can help interpret what the number means. However, screening tests do not give a “positive” or “negative” result. In addition, these screens can miss some pregnancies where a condition is actually present (“false negatives”). They can also incorrectly show high chances for a condition when none exists (“false positives”).3 These screens can accurately suggest a higher chance of Down syndrome about 50-95% of the time, depending on the methods used and the condition, and they show “false positives” in about 5% of cases.4 No blood test currently offered can give a definite “yes” or “no” answer even though they can often identify those with increased chances.

Ultrasound screens for some genetic conditions as well. For instance, a measurement at the back of the fetal neck is known as the nuchal translucency (NT) and if increased in thickness in the first trimester, there is a higher chance of a chromosome condition such as Down syndrome. In the second trimester, there are measurements and findings on the ultrasound that can show an increased or decreased chance of a chromosome condition, but there is no way to find a prenatal chromosome condition for sure based on ultrasound findings alone. For instance, a fetus with shorter than average femur (thigh bones) on ultrasound may have an increased chance of Down syndrome. However, most babies with shorter than average thigh bones do not have Down syndrome, and some babies with Down syndrome have average length thigh bones. Ultrasound is a very good tool for detecting most neural tube defects such as spina bifida. In fact, greater than 98% of neural tube defects can be seen on ultrasound in the second trimester.5

Non-Invasive Prenatal Screening Tests

There are also newer non-invasive prenatal screening tests that can be done after 9 or 10 weeks. These tests have the highest detection rates among all screening tests, specifically for women who have already learned their pregnancy has greater chances for a genetic condition. These tests look at pieces of genetic material in the woman’s blood to find chromosome conditions, such as Down syndrome, trisomy 13 and 18, and, sometimes, sex chromosome and other genetic conditions. These tests are still considered screening tests because false negatives and false positives do occur. The accuracy is improved but varies depending on the condition. So, diagnostic testing is recommended for those who want to know for sure. In addition, these tests do not detect many other genetic conditions or factors that might be present in a pregnancy. These tests also might not be as reliable in some situations, such as with twins or multiples.4 The results given by the testing companies can be very different from each other and should be reviewed with the patient by a medical professional.

Young Black woman with Turner syndrome holding books and walking down an outdoor corridor.

Diagnostic Tests

Diagnostic testing is used to confirm a diagnosis and is over 99% accurate for chromosome conditions. These tests also detect other genetic conditions that are not found by screening tests. Diagnostic testing includes amniocentesis or chorionic villus sampling (CVS), which take a sample of the amniotic fluid or the placenta. An amniocentesis is usually available between 15-20 weeks, and a CVS is typically available after 9 weeks of pregnancy. These tests have a small risk of miscarriage (usually less than 1%).6,7 For more detailed information about testing, patients can visit the Genetic Support Foundation.

Asian woman holding a toddler boy with Prader-Willi syndrome.

Chromosome Conditions

While prenatal testing offers more information about the pregnancy, it can also lead to many more questions such as: What do these screening results or this diagnosis mean? What kind of life does a person with this diagnosis live? Where can reliable information about this condition be found?

Sometimes people find incorrect or out-of-date information when trying to learn about different conditions. The outcomes and attitudes about many disabilities have changed greatly in recent years. This means patients need current information about genetic conditions so that they can make informed choices about the pregnancy and find any needed services or resources.

Each of the chromosome conditions that prenatal tests look for are very different. Pregnancies with Trisomy 13 and Trisomy 18 have much higher chances for miscarriage and stillbirth. Babies who have these conditions often have serious medical and neurological issues, and only about 10% live past the first year.8 People with Down syndrome typically have mild to moderate intellectual disabilities and some medical issues that can be treated with good health care. They are usually active members of their communities and live an average of 60 years.9 On the other hand, the effect of an extra or missing sex chromosome (such as Turner syndrome, Klinefelter syndrome, and 47,XXX) can be so mild that people might not even know they have a chromosome condition.

For current information about prenatally diagnosed conditions, patients can speak with their obstetric medical care provider or genetic counselor and search the websites of national organizations found below.

Genetic counselors and medical geneticists can be found by asking for a referral or searching online at the National Society for Genetic Counselors or the American College of Medical Genetics and Genomics.

Prenatal screening and testing are constantly changing to include more conditions. Therefore, it is important for patients to talk to their medical providers about what conditions are included in the different testing options.

Young man with Down syndrome holding a chihuahua. He is sitting on a bench next to a young woman holding a large dog on a leash.

Trisomy Conditions

Typically, an individual has 23 pairs of chromosomes with 46 chromosomes all in all. The first 22 pairs of chromosomes are numbered according to size, and the last pair are known as the sex chromosomes as they usually determine gender. Most babies inherit a set of 23 chromosomes from the mother and a set of 23 chromosomes from the father.

More information about chromosomes can be found at MedlinePlus.

A trisomy chromosome condition occurs when a person receives extra chromosome material on one of those chromosomes. The most common trisomy chromosome conditions include Down syndrome (also known as Trisomy 21), Trisomy 18, and Trisomy 13.

The chance for a pregnancy to be affected by one of these conditions increases as the age of the mother increases as shown by this chart of maternal-age prevalence, but everyone has some chance to have a baby with a trisomy condition. It is rare for these conditions to run in a family (be inherited), and there is nothing that can be done to cause or prevent a trisomy condition.

Down syndrome or Trisomy 21

Down syndrome is caused by an extra or partial copy of chromosome 21 and affects about 250,000 people in the US.9 The overall miscarriage rate for Down syndrome between a diagnosis by CVS and term is approximately 32%, and 25% between amniocentesis and term.10 Most people with Down syndrome have mild to moderate intellectual disabilities and actively participate in their families, communities, and schools with support. Babies with Down syndrome also have low muscle tone and increased chances for certain health conditions, such as heart defects and digestive issues. However, most health issues can be successfully treated or managed, and the average life expectancy is about 60.9

Trisomy 13 (Patau syndrome) and Trisomy 18 (Edward’s syndrome)

Trisomy 18 is caused by an extra copy of chromosome 18 and impacts about 1 in 5,000 live births.11 About 72% of Trisomy 18 pregnancies result in miscarriage or stillbirth between 12 weeks to term.8 Less than 10% of babies born with Trisomy 18 currently survive past the first year12, but some children and adults can live longer with intensive medical care. Individuals with Trisomy 18 also have significant intellectual disabilities.

Trisomy 13 occurs in about 1 in 16,000 live births and is caused by an extra copy of chromosome 13. About half of Trisomy 13 pregnancies result in miscarriage or stillbirth between 12 weeks to term,13 and about 5-8% of babies with Trisomy 13 survive past the first year, usually with intensive medical care and significant intellectual disability.14

While those who survive with Trisomy 13 and 18 have profound disabilities, they can accomplish delayed milestones and build meaningful relationships with friends and family. Families affected by Trisomy 13 and 18 often say that they want the opportunity to determine how little or how much medical care their fetus or child receives, and they want access to support services and grief counseling as needed.14

Trisomy 16

Trisomy 22

White girl with Turner syndrome dribbling a soccer ball with her knee on an outdoor soccer field.

Sex Chromosome Conditions

Usually, a female has two copies of the X chromosome (XX) while males typically have an X and Y chromosome (XY). The last pair of chromosomes are known as the “sex chromosome” and typically determines gender. Any variation in the number of sex chromosomes (extra or missing X or Y chromosomes) causes a sex chromosome condition.

X and Y chromosome variations affect about 1 in 500 people. Some sex chromosome conditions never show any signs throughout a lifetime. The life expectancy for people with sex chromosome conditions is usually the same as the rest of the population though there is greater risk of miscarriage for some sex chromosome conditions, such as Turner syndrome.15 Individuals with a sex chromosome condition may have some symptoms that include developmental delays, treatable psychological issues, and some physical or growth differences. For example, a woman with three copies of the X chromosome may be taller than her peers but may not have any other developmental or physical signs of an extra chromosome. Most individuals with sex chromosome conditions do not have intellectual disabilities.16

Turner Syndrome

Turner Syndrome (TS) occurs when there is only one copy of the X chromosome and affects about 1 in 2,500 newborn girls and affects each individual differently. Some have features noticed at birth while others are not apparent until adolescence or adulthood. Although females with TS are usually infertile and shorter than average, most have normal sexual function and can grow and develop typically with hormone therapy. Other health conditions may include treatable heart defects (30-50%), minor physical features, skeletal differences, or kidney issues. Women with TS usually have typical intelligence and have an increased chance for learning disabilities that can be treated with support and early education. Most adults with TS function well and live independently.17,18

Klinefelter Syndrome

Klinefelter syndrome or 47, XXY, is found in about 1 one in 600 males. Some individuals with Klinefelter syndrome have different degrees of learning difficulties and developmental delays. Most men with Klinefelter syndrome are taller than average and are infertile. Many men who have Klinefelter syndrome do not know they have it until undergoing an evaluation for infertility. Recent advances in assisted reproduction technology have allowed for the possibility of those with Klinefelter syndrome to father biological children.16

Neural Tube Defects

Spina Bifida

Young boy with Jacobsen syndrome standing and typing at a desk.

Microdeletions

22q deletion syndrome

5p (Cri-du-chat syndrome)

15q (Prader-Willi/Angelman Syndrome)

1p36 deletion syndrome

4p (Wolf-Hirschhorn syndrome)

8q (Langer-Giedion syndrome)

11q (Jacobsen syndrome)

Conclusion

For current information about prenatally diagnosed conditions, patients can speak with their obstetric medical care provider or genetic counselor and search the websites of national organizations found below.

Genetic counselors and medical geneticists can be found by asking for a referral or searching online at the National Society for Genetic Counselors or the American College of Medical Genetics and Genomics.

Prenatal screening and testing are constantly changing to include more conditions. Therefore, it is important for patients to talk to their medical providers about what conditions are included in the different testing options.

End Notes

  1. Update on overall prevalence of major birth defects—Atlanta, Georgia, 1978-2005. MMWR Morb Mortal Wkly Rep 2008;57:1-5.
  2. Allison SJ, Stafford J, Dilly OC. The effect of stress and anxiety associated with maternal prenatal diagnosis on feto-maternal attachment. Anumba. BMC Women’s Health. July 2011;11:33.
  3. ACOG Practice Bulletin No. 77, January 2007. Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol 2007;109:217–27.
  4. Anthony R. Gregg, S.J. Gross, R.G. Best, K.G. Monaghan, K. Bajaj, B.G. Skotko, B.H. Thompson, M.S. Watson. ACMG statement on noninvasive prenatal screening for fetal aneuploidy. Genet Med 15: 395-398; April 4, 2013; doi:10.1038/gim.2013.29
  5. Morrow R.J., McNay M.B., Whittle M.J. Ultrasound detection of neural tube defects in patients with elevated maternal serum alpha-fetoprotein. Obstet Gynecol 1991 Dec;78(6):1055-7.
  6. Caughey AB, Hopkins LM, Norton ME. Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstet Gynecol 2006;108:612-6. (Level II-2)
  7. Alfirevic Z, Mujezinovic F, Sundberg K. Amniocentesis and chorionic villus sampling for prenatal diagnosis. Cochrane Database of Systematic Reviews 2003, Issue 3. Art. No.: CD003252. DOI: 10.1002/14651858.CD003252.
  8. Morris JK, Savva GM. The risk of fetal loss following a prenatal diagnosis of trisomy 13 or trisomy 18. Am J Med Genet A. 2008 Apr 1;146(7):827-32. doi: 10.1002/ajmg.a.32220.
  9. de Graaf, G., Buckley, F., Skotko, B.G. (2015). Estimates of the live births, natural losses and elective terminations with Down syndrome in the United States. American Journal of Medical Genetics Part A, 167A:756–767.
  10. Savva, G. M., Morris, J. K., Mutton, D. E., & Alberman, E. (2006). Maternal age-specific fetal loss rates in Down syndrome pregnancies. Prenatal Diagnosis, 26(6), 499–504.
  11. http://ghr.nlm.nih.gov/condition/trisomy-18
  12. Rasmussen et al. Population-based analyses of mortality in trisomy 13 and trisomy 18. Pediatrics. 2003 Apr;111(4 Pt 1):777-84.
  13. http://ghr.nlm.nih.gov/condition/trisomy-13
  14. http://trisomy.org/wp-content/uploads/2013/04/Common-Problems-of-Babies-with-Trisomy-18-or-Trisomy-13.pdf
  15. http://ghr.nlm.nih.gov/condition/turner-syndrome
  16. http://www.genetic.org/Portals/0/Public/Brochures/KS&A%20brochure%20general%20WEB.pdf
  17. http://turnersyndrome.org/files/resources/prenatal-diagnosis-xo-212.pdf
  18. http://turnersyndrome.org/files/resources/clinical-practice-guidelines-turner-syndrome-210.pdf