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First Trimester
Down Syndrome Screen

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Formal name: First Trimester Screen [Pregnancy-associated plasma protein-A (PAPP-A), human chorionic gonadotropin (hCG) as either free beta subunit or total hCG, and nuchal translucency ultrasound]

At a Glance

Why Get Tested?

To assess the risk of a fetus having a chromosomal abnormality, such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18)

When to Get Tested?

Usually between 11 and 14 weeks of pregnancy

Sample Required?

A blood sample drawn from a vein in your arm or a blood spot collected on special paper from a finger stick; a nuchal translucency requires a special ultrasound exam.

Test Preparation Needed?

You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.

The Test Sample

What is being tested?

The first trimester screen is a combination of two blood tests and a special ultrasound that are used to assess a pregnant woman's risk of carrying a baby with Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Performing and evaluating them together, along with the woman's age, increases both the sensitivity and specificity of the screening results.

  • Pregnancy-associated plasma protein A (PAPP-A) is a protein produced first by the outer layer of the developing pregnancy (trophoblast) and then by the growing placenta. During a normal pregnancy, levels of this protein increase in the pregnant woman's blood until delivery.
  • Human chorionic gonadotropin (hCG) is a hormone produced by the developing pregnancy and then produced in large quantities by the placenta. Either free beta subunit or total hCG can be used in first trimester screening. Levels of both usually rise rapidly in the pregnant woman's circulation for the first 8 to 10 weeks, then decrease and stabilize at a lower level for the remainder of the pregnancy.
  • Nuchal translucency is measurement made by ultrasound. The ultrasonagrapher measures the fluid collection between the spine and the skin at the nape of the fetus's neck. It is a procedure that requires a specially trained radiologist, proper alignment of the fetus, and careful measurement. It is not a routine ultrasound, and it is not a procedure that is available at every hospital or health facility.

If the results of first trimester screening are cause for concern, further screening tests (e.g., quad screen) or diagnostic tests (e.g., amniocentesis or chorionic villus sampling (CVS)) may be recommended.

How is the sample collected for testing?

Blood is drawn from a vein in the woman's arm or collected from a finger stick. The nuchal translucency ultrasound may be performed from outside the abdomen (transabdominally) or the probe may be inserted into the vagina (transvaginally).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

ACOG. Screening for Birth Defects. Available online at http://www.acog.org/~/media/For%20Patients/faq165.pdf?dmc=1&ts=20130213T1410405774 through http://www.acog.org. Accessed February 2013. 

CDC. Birth Defects, Diagnosis. Available online at http://www.cdc.gov/ncbddd/birthdefects/diagnosis.html through http://www.cdc.gov. Accessed February 2013. 

MayoClinic.com. First trimester screening. Available online at http://www.mayoclinic.com/health/first-trimester-screening/MY00126 through http://www.mayoclinic.com. Accessed February 2013. 

March of Dimes. Birth defects: Down syndrome. Available online at http://www.marchofdimes.com/baby/birthdefects_downsyndrome.html through http://www.marchofdimes.com. Accessed February 2013. 

Trisomy 18 Foundation: What is Trisomy 18? Available online at http://www.trisomy18.org/site/PageServer?pagename=whatisT18_whatis through http://www.trisomy18.org. Accessed February 2013. 

LabCorp. First Trimester Maternal Serum Screening. PDF available for download at http://www.ptow.com/x_upload/files/Maternal%20Serum%20Screening.pdf through http://www.ptow.com. Accessed February 2013. 

Sources Used in Previous Reviews

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 710-713.

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Newberger, D. (2000 August 15). Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20000815/825.html through http://www.aafp.org. Accessed on 4/15/07.

Qin, Q., et. al. (2002). Point-of-Care Time-resolved Immunofluorometric Assay for Human Pregnancy-associated Plasma Protein A: Use in First-Trimester Screening for Down Syndrome. Clinical Chemistry 48:473-483 [On-line journal]. Available online at http://www.clinchem.org/cgi/content/full/48/3/473 through http://www.clinchem.org. Accessed on 4/15/07.

(2007 January). Maternal Blood Screening for Birth Defects. March of Dimes [On-line information]. Available online at http://www.marchofdimes.com/printableArticles/14332_1166.asp through http://www.marchofdimes.com. Accessed on 4/15/07.

Barclay, L. and Lie, D. (2007 January 4). New Guidelines Recommend Universal Prenatal Screening for Down Syndrome. Medscape Medical News [On-line information]. Available online at http://www.medscape.com/viewarticle/550256_print through http://www.medscape.com. Accessed on 4/10/07

Cleary-Goldman, J. and Malone, F. (2005 September 26). Advances in Prenatal Diagnosis. Medscape from Appl Radiol 2005;34(9): 8-18 [On-line information]. Available online at http://www.medscape.com/viewarticle/513126_print through http://www.medscape.com. Accessed on 4/10/07.

Cole, D.S. (© 2005). Advances in First-Trimester Prenatal Diagnosis: Combined, Integrated, or Amniocentesis? Medscape Conference Coverage: American College of Obstetricians and Gynecologists 53rd Annual Clinical Meeting [On-line information]. Available online at http://www.medscape.com/viewprogram/4078_pnt through http://www.medscape.com. Accessed on 4/10/07.

American College of Radiology and the Radiological Society of North America. Radiology Info, Obstetric ultrasound (online information, accessed June 2007). Available online at http://www.radiologyinfo.org/en/info.cfm?PG=obstetricus through http://www.radiologyinfo.org.

Malone FD, et al. First Trimester or Second Trimester Screening or Both for Down Syndrome. New England Journal of Medicine. 2005;353: 2001-2011.

ACOG News Release (January 2, 2007). New Recommendations for Down Syndrome Call for Screening of All Pregnant Women. Available online at http://www.acog.org/from_home/publications/press_releases/nr01-02-07-1.cfm through http://www.acog.org.

ACOG Practice Bulletin Clinical Management Guidelines for Obstetrician-Gynecologists. Number 77, January 2007. Screening for Fetal Chromosomal Abnormalities. Obstetrics & Gynecology 2007; 109: 217-227.

(© 2010). Screening for Birth Defects. ACOG [On-line information]. Available online at http://www.acog.org/publications/patient_education/bp165.cfm through http://www.acog.org. Accessed June 2010.

(2008 November). Maternal Blood Screening for Birth Defects. March of Dimes [On-line information]. Available online at http://www.marchofdimes.com/professionals/14332_1166.asp through http://www.marchofdimes.com. Accessed June 2010.

Mayo Clinic Staff. (2008 July 2). First trimester screening. MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/first-trimester-screening/MY00126 through http://www.mayoclinic.com. Accessed June 2010.

Ekelund, C. et. al. (2008 December 08). Impact of a new National Screening Policy for Down's Syndrome in Denmark: Population Based Cohort Study. Medscape from BMJ. 2008;337(2547) [On-line information]. Available online at http://www.medscape.com/viewarticle/584622 through http://www.medscape.com. Accessed June 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 757-759.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 252-259, 886-889.

(February 2006) American Pregnancy Association. First Trimester Screen. Available online at http://www.americanpregnancy.org/prenataltesting/firstscreen.html through http://www.americanpregnancy.org. Accessed September 2010.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC Pp 416-418.

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