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Screening for Down Syndrome

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pregnant woman talking to a doctor

Pregnancy and childbirth can be the most exciting times in a woman’s life. However, they can also be the most stressful, as many parents have questions about their unborn baby’s health.

Prenatal genetic screening can identify potential genetic risks and birth defects before delivery. A noninvasive screening test for Down syndrome has recently become available, which is more accurate than tests currently being used.

Down syndrome, also called trisomy 21, occurs when there is an extra chromosome 21 present in cells of the fetus. Children with this extra chromosome have mental retardation, physical characteristics of Down syndrome and about 40 percent will have serious heart problems. “Today, screening tests used during pregnancy have the ability to determine the risk of having a child with a genetic disorder like Down syndrome as early as 10 weeks gestation,” says Michael Mennuti, MD, chief of reproductive genetics at Penn Medicine.

What Screening Tests Tell You

Screening tests don’t diagnose the condition, but inform parents about the likelihood or chance of having a child with a specific genetic disorder. Screening for Down syndrome can be performed as early as 11 to 14 weeks of pregnancy with a first trimester ultrasound and blood test. Screening can also be performed between 15 and 20 weeks by a blood test referred to as the multiple marker serum screening test. Some women may elect to use a combination of first and second trimester screening tests, known as the sequential screen.

“Each of these tests uses a mother’s age, results of the blood test and ultrasound measurements of the fetus to provide her with an adjusted risk for having a child with Down syndrome,” says Dr. Mennuti. “While these tests are good, they cannot detect every case of Down syndrome; they offer detection rates from 80 to 90 percent.”

There is a new screening test for Down syndrome, trisomy 13 and trisomy 18 currently offered at Penn Medicine which is more sensitive.

“This new screening test, offered to women with the highest risk for having a child with Down syndrome, is 99 percent accurate in screening for Down syndrome,” says Dr. Mennuti. “This means if 100 women were to have this test, 99 of the 100 would be detected. The test can be performed as early as 10 weeks of pregnancy.”

The screening test measures the amount of DNA from chromosome 21 in a pregnant woman’s blood. Cells of fetuses with Down syndrome have an extra 21st chromosome so they release more chromosome 21 DNA into the maternal blood when broken down. This causes a higher than expected amount of chromosome 21 DNA in the mother’s blood which can then be measured. The results are usually available about 10 days after the sample is drawn.

Parents who learn their child is at high risk for an inherited chromosomal condition, such as trisomy 21 have the option to pursue an invasive diagnostic test like amniocentesis and chorionic villus sampling (CVS).

Currently, this new test is only available to women at highest risk for having a child with Down syndrome. Physicians and researchers anticipate that this will change in the future as the test is validated in a low risk population.

Who should get a screening test?

Screening tests for Down syndrome are offered to every woman during pregnancy. Your decision to have a screening test is a personal one. The following women are at a higher risk for having a child with a chromosomal disorder such as trisomy 21 and should consider speaking with a genetic counselor to learn more:

  • 35 or older when they deliver
  • a previous child or fetus with Down syndrome or a chromosome abnormality
  • a family history or parent with a chromosomal abnormality
  • fetal anomalies detected on ultrasound

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