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First Trimester Down Syndrome Screen

Last updated Nov. 13, 2018

Approved by: Krish Tangella MD, MBA, FCAP

Darryl Leja - NHGRI

An individual with Down syndrome inherits all or part of an extra copy of Chromosome 21.


What are the other Names for this Test? (Equivalent Terms)

  • First Trimester Screen (with Nuchal Translucency Ultrasound, Pregnancy-Associated Plasma Protein-A (PAPP-A), and Human Chorionic Gonadotropin (hCG))

What is First Trimester Down Syndrome Screen Test? (Background Information)

  • The human cell contains 23 pairs of chromosomes. Down syndrome is caused by the presence of three copies of the chromosome 21 (trisomy 21). This disorder is characterized by mental retardation and may be associated with presence of cardiac problems, thyroid dysfunction, increased risk of blood cancers, and a reduced life span
  • The First Trimester Screen is a series of tests designed to identify any possible chromosomal abnormality, such as Down syndrome, or Edwards syndrome (presence of three copies of chromosome number 18)
  • The First Trimester Screen, which is performed during 11-13 completed weeks of pregnancy, includes:
    • Test for pregnancy associated plasma protein A (PAPP-A): A blood test, PAPP-A is a protein produced by the trophoblast (the outer layer of the developing fetus in the early stages) and then by the developing placenta (the tissue that serves to provide nutrients to the developing baby from the mother and helps in waste elimination). The level of PAPP-A, increases in the mother’s blood, until delivery. In babies with Down syndrome, the levels tend to be lower
    • Test for human chorionic gonadotropin (hCG): A Blood test, the hCG (total or the beta sub-unit) is a hormone that is produced by the syncytiotrophoblast (precursor to placenta) and the placenta. The hCG levels in the mother’s blood increase during the first 8-10 weeks of pregnancy and thereafter stabilizes to a lower level. The hCG levels, remain at a higher level, in those with Down syndrome
    • Test for nuchal translucency, performed using ultrasound. This estimates the amount of fluid present between the spine and the skin on the back of the fetus’ neck. This is increased, in case of Down syndrome

What are the Clinical Indications for performing the First Trimester Down Syndrome Screen Test?

The First Trimester Screen is performed during 11-13 completed weeks of pregnancy, in order to find out the risk of having a baby with the chromosomal abnormality (i.e. Down syndrome or Edwards syndrome).

How is the Specimen Collected for First Trimester Down Syndrome Screen?

Sample required: Blood (for pregnancy associated plasma protein A and human chorionic gonadotropin)

Process:

  • Blood sample is drawn through a needle inserted into the vein (arm), or obtained through a finger prick
  • Nuchal translucency is determined using ultrasound and the probe can be placed, either on the belly region (transabdominal), or inserted into the vagina (transvaginal)

Preparation required: None, for the blood test. But, it is necessary to have a full bladder for the ultrasound examination.

What is the Significance of the First Trimester Down Syndrome Screen Result?

  • The individual test results of PAPP-A, hCG, and the nuchal translucency ultrasound, are combined and used to determine the risk of having a baby with a chromosomal abnormality
  • A positive screen indicates the risk of such a chromosomal problem and warrants further testing, such as a chorionic villus sampling in the first trimester, or amniocentesis in the second trimester to make a definitive diagnosis. It has to be noted that the screening tests are not diagnostic in nature
  • It is to be noted that while the first trimester screen can approximately identify 85% of pregnant women, with Down syndrome babies, and 75% of those with Edward syndrome babies; in 5% of the cases, the test results maybe false positive (meaning that the pregnancy is presently normal)

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • It has been found out that the first trimester screening is the most accurate, non-invasive screening method available (New England Journal of Medicine, November 2005)
  • A positive screen result indicates a 1/100 to 1/300 chance of carrying a baby with any one of the chromosomal abnormality

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

What are some Useful Resources for Additional Information?


References and Information Sources used for the Article:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Dec. 30, 2013
Last updated: Nov. 13, 2018