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Screening for and Diagnosing Chromosomal Abnormalities

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The purpose of screening is early diagnosis and treatment. Although there is no treatment for chromosomal abnormalities, there are numerous screening procedures that can test for problems. Finding out about potential problems in your baby can help you decide whether to undergo more invasive testing. It can also help you prepare for the care your infant will need. The option of terminating the pregnancy can also be discussed.

You should talk with your doctor or midwife about whether you want to be screened for chromosomal abnormalities in your baby. Several screening options are available:

Maternal Blood Screening

This procedure identifies pregnancies at a higher than average risk of birth defects and chromosomal abnormalities. You'll have blood drawn at 15-18 weeks of pregnancy. The blood sample is tested for levels of substances in your blood. If the levels are abnormal, you may be offered ultrasound and/or amniocentesis to provide more detailed information. A screening test is not diagnostic and does not mean that there is an abnormality. A positive screening test result simply means that further testing may be needed because of increased risk.

AFP (Alpha-Feto Protein) Screen

The AFP screen is the most commonly available prenatal screening test for birth defects. AFP is a natural substance produced by the fetus’s liver and yolk sac. It normally enters the mother’s bloodstream during pregnancy. Your levels of AFP are higher than normal if the fetus has a neural tube defect. A neural tube defect occurs when the vertebrae that normally enclose the spinal cord fails to develop. AFP leaks out of the open spinal column into your blood. Potential problems include spina bifida and problems with brain development. A lower than normal AFP level can indicate Down syndrome or another chromosomal defect.

Performed at 16-18 weeks into pregnancy, you’ll have blood drawn from your arm. It is a safe test and you should have the results within one week.

Although common, this screening test is not very accurate. There is a very high rate of false results, where the positive high or positive low test results do not indicate a problem with the fetus. If your AFP test is abnormally high or abnormally low, your doctor may recommend further tests, such as an ultrasound and/or amniocentesis.

Triple Screen

The triple screen is a maternal blood screening test that looks for three specific substances: human chorionic gonadotropin (hCG), which is a hormone produced within the placenta; alpha-fetoprotein (AFP); and estriol, which is an estrogen produced by both the fetus and the placenta. For more information on these substances, see the section below.

A triple screen involves having a blood sample drawn usually between the 15th and 17th week of pregnancy.

This test is recommended particularly for women who:

  • Have a family history of birth defects
  • Are 35 years or older—The cut-off age of 35 is somewhat arbitrary, and patients below and above 35 should discuss the risks with their doctor and genetic counselor.
  • Used potentially harmful medications or drugs during pregnancy
  • Have diabetes and use insulin
  • Had a viral infection during pregnancy
  • Have been exposed to high levels of radiation

The test looks for unusual levels of hCG, AFP, and estriol. The results are combined with the mother's age and ethnicity in order to assess probabilities of potential genetic disorders. Abnormal levels of hCG and estriol, for example, may indicate that the developing baby may have chromosome abnormalities.

Although the primary reason for conducting the triple test is to screen for genetic disorders, the test can sometimes identify heart and digestive system defects, the existence of twins, and risks of premature birth, pre-eclampsia , and miscarriage.

Quad Screen

The quad screen test is a maternal blood screening test that looks for four specific substances: hCG, AFP, estriol, and inhibin-A, which is the fourth test that was added to the triple screen.

  • hCG : Human chorionic gonadotropin is a hormone produced within the placenta.
  • AFP : Alpha-fetoprotein is a protein that is produced by the fetus.
  • Estriol : Estriol is an estrogen produced by both the fetus and the placenta.
  • Inhibin-A : Inhibin-A is a protein produced by the placenta and ovaries.
Condition: hCG: AFP: Estriol: Inhibin-A
Down Syndrome High Low Low High
IUGR, Preterm Birth, Stillbirth High
Multiple Gestation High High High
Open Neural Tube Defects Normal High Normal
Trisomy 13 Low Normal
Trisomy 18 Low Low Low

The screen is essentially the same as the triple screen. The accuracy, though, of identifying pregnancies at risk for Down Syndrome is higher through the evaluation of inhibin-A levels. The false positive rate of the test is also lower.

Ultrasound

Ultrasound is an imaging technique that uses high-frequency sound waves to see inside the body. An ultrasound given 10-14 weeks into your pregnancy can help your physician evaluate the neck fold, which is larger in a fetus with Down syndrome or Turner syndrome.

Amniocentesis

Performed at 15-18 weeks of pregnancy, your doctor inserts a thin needle into your abdomen. Using ultrasound as a guide, a small amount of the fluid surrounding the fetus is extracted for testing. There is a small risk (1 in 400) of miscarriage.

This procedure is generally recommended for women who:

  • Are over 35 years of age
  • Have a previous child with a birth defect that can be diagnosed by amniocentesis
  • Have a family history of a genetic disorder
  • Have an abnormal triple screen test result

This test tells a lot about the genetic makeup of your baby and reveals more common genetic defects. It takes one to two weeks to get the results on chromosomal abnormalities and the baby's sex—the result is the actual unique karyotype of the fetus. The karyotype is the 23 pairs of chromosomes, in detail. Other results are available the next day.

Chorionic Villus Sampling (CVS)

This is a more invasive procedure and is usually performed between the 10th to 12th week of pregnancy. The doctor inserts a thin tube through the vagina and cervix and extracts a tiny tissue sample from the inside of the uterus. The doctor can also insert the needle through the abdomen into the uterus to extract tissue from the chorionic villi, which are finger-like projections of tissue that surround the fetus in the early weeks and ultimately form the placenta.

There is a small risk of miscarriage (1 in 100), which is higher than the risk for amniocentesis. Vaginal bleeding is also common. This procedure provides more genetic and biochemical information than amniocentesis, can be performed earlier in pregnancy, and provides results more quickly.

This test is recommended for women who:

  • Are over 35 years of age
  • Have a child with a genetic defect
  • Have a family history of a genetic disorder
  • Have an abnormal first trimester screening

Revision Information

  • Reviewer: Kari Kassir, MD
  • Review Date: 05/2014 -
  • Update Date: 05/28/2014 -
  • Chromosome abnormalities. National Human Genome Research Institute website. Available at: http://www.genome.gov/11508982 . Updated October 13, 2011. Accessed July 24, 2013.

  • Leshin L. Prenatal screening for Down syndrome. The Down Syndrome: Health Issues website. Available at: http://www.ds-health.com/prenatal.htm . Accessed July 25, 2013.

  • Prenatal testing. The American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting . Accessed July 25, 2013.