Screening for Down Syndrome

May 5, 2019

Screening for Down Syndrome

It is typical for women who are pregnant to undergo a series of prenatal screenings to find out if their baby is at increased risk for certain conditions. One of the conditions commonly tested for is Down Syndrome.

Down Syndrome is a chromosomal disorder that affects about 1 in 500-1000 babies. It is caused by a genetic condition wherein there is an extra whole or partial 21st chromosome. Down Syndrome is not a product of a particular culture, socioeconomic status, or ethnicity—it affects people from all over the world and from all backgrounds.

The American College of Obstetricians and Gynecologists recommends that all pregnant women do a prenatal test for Down Syndrome before 20 weeks of pregnancy. There are two types of prenatal tests—screening tests and diagnostic tests.

Screening Tests for Down Syndrome

In general, screening tests will not provide a 100% final answer as to whether or not a baby has Down Syndrome, but it will identify if there is increased risk. Screening tests are completely safe for both the mother and the baby, and there are three types.

A combined first trimester screening is done between 9 and 13 weeks (sometimes between 10 and 14 weeks) into the pregnancy. First, a blood test screens for hormonal changes that suggest problems with the baby’s chromosomes. Second, a health professional will do an ultrasound scan to measure the thickness of the fluid behind the baby’s neck. Babies with Down Syndrome will have a thicker measurement.

Non-invasive prenatal testing is a sensitive blood test performed after week 10 of pregnancy (usually between 15 and 20 weeks). Blood is taken from the mother, and the baby’s DNA is analyzed.

For mothers who have missed the deadlines for the combined first trimester screening test, there is another option called Second Trimester Maternal Serum Screening. Performed between 14 and 18 weeks into the pregnancy, this blood test also looks for hormones that indicate Down Syndrome.

Diagnostic Tests

Diagnostic tests are usually only performed for women who are at increased risk (over the age of 40), have a family history of genetic conditions, or who have had either positive or inconclusive results from a screening test. Diagnostic tests can increase the chance of miscarriage, so they are not performed unless necessary. There are two diagnostic tests that can confirm a Down Syndrome diagnosis.

The first test, known as amniocentesis, is the most widely used method to identify chromosomal problems, including Down Syndrome. The test is performed between weeks 15 and 20 of pregnancy. Using ultrasound as a guide, a doctor inserts a needle into the mother’s abdomen and withdraws a small amount of amniotic fluid. Cells from the fluid are analyzed for their chromosomal makeup, which identifies most chromosomal disorders.

The second test, Chorionic Villus Sampling (or CVS) also detects chromosomal problems like Down Syndrome. Unlike amniocentesis, CVS can be performed as early as 10 weeks into pregnancy. With ultrasound guidance, a doctor inserts a needle either through the mother’s abdomen or via a catheter through the cervix. A sample of tissue from the placenta is extracted, cultured, and analyzed for chromosomal problems.

Sources:

  • https://www.pregnancybirthbaby.org.au/screening-for-down-syndrome

  • https://www.ucsfhealth.org/education/down_syndrome/

  • https://www.uptodate.com/contents/should-i-have-a-screening-test-for-down-syndrome-during-pregnancy-beyond-the-basics

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This post does not provide medical advice. See Additional Information.

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