First Trimester Screening for Down Syndrome and Trisomy 18

What is Down Syndrome?

Down Syndrome is a chromosomal disorder where individuals have a distinct physical appearance and varying degrees of mental retardation. They are also at increased risk for certain birth defects, such as congenital heart defects.

Typically, there are 46 chromosomes in each of our cells. Individuals with Down Syndrome have a total of 47 chromosomes in their cells. Any woman can have a baby with Down Syndrome, regardless of her age, health or family history. However, the risk gradually increases with a woman’s age. Approximately 1 in 800 babies are born with Down Syndrome each year.

Trisomy 18 is also a chromosome abnormality. Babies with Trisomy 18 have many serious mental and physical disabilities including mental retardation, heart defects, and feeding and growth disorders. About 1 in 8000 babies is born with Trisomy 18 in the United States. As with Down Syndrome, any woman is at risk to have a child with Trisomy 18, but the risk increases with the age of the mother.

What is the test looking at?

Screening is a combination of a first trimester ultrasound and a blood test performed between the 11th and 13th week 6th day of pregnancy. Accurate dates for the pregnancy will be determined by measuring the crown rump length. Then, the ultrasound will measure the skin thickness on the back of the fetal neck. Increases in this measurement have been associated with fetal chromosome abnormalities.

During this same appointment, a blood sample will be obtained and analyzed for two proteins which are normally found in the blood of all pregnant women. These two proteins change in an abnormal pregnancy. The results of the ultrasound exam will be combined with the results of the blood test to estimate a specific risk for Down Syndrome and Trisomy 18.

It is important to note that first trimester screening is specific for Down Syndrome and Trisomy 18. It does not provide risk assessment for other chromosome or genetic disorders. If you are 35 years old or older, you may consider an amniocentesis to rule out other more rare chromosome disorders.

What do the results mean?

It is estimated that 90% of fetuses with Down Syndrome and 98% of babies with Trisomy 18 will have a positive first trimester screen.

A screening test DOES NOT provide a diagnosis, but it predicts the likelihood of a problem. This test is used to put a pregnancy into risk categories. If a woman’s first trimester screening result is abnormal, it DOES NOT necessarily mean that the baby has a chromosome abnormality, but rather that the risk is higher than average.

The nuchal translucency (NT) is the thickness of the skin on the back of the neck.

Follow up testing to consider

If a pregnancy is at risk for a chromosome abnormality, a woman can consider amniocentesis for a diagnostic result.

Regardless of the first trimester screening result, if you elect not to have an amniocentesis, you may consider the AFP test during your second trimester to check for open neural tube defects,  like spina bifida.

Learn more

Genetic Counseling Services translates scientific knowledge into practical information that you can use in making family decisions. For more information about genetic counseling, click here or call 918-293-6200.

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