First Trimester Genetic Screening

During prenatal genetic counseling, you’ll learn what genetic tests might benefit a woman of your age, medical history, and family history. Ask about a simple first-trimester genetic screening that measures the risk of Down syndrome and other chromosomal abnormalities.

What is a first trimester genetic screen test? 

This test combines an ultrasound and a finger stick blood test. These tests take place sometime between 11 weeks to 13 weeks 6 days from the beginning of pregnancy.

During the ultrasound, we'll measure the length of the embryo and its heart rate. We'll also measure the space between the back of the neck and the skin, called an NT. The blood test checks for two chemicals. The results of the blood test and the NT together with your age, give a number that measures the risk for Down syndrome and trisomies 18 and 13. 

What are the benefits?

This test helps identify babies who may have Down syndrome or other abnormal chromosomes. It gives information earlier in the pregnancy than any other screening test. The result is extra time to decide whether to have other tests.

What is considered abnormal? 

It is important to remember that this is a screening test. It does not give a diagnosis. It only gives an estimate of the risk for Down syndrome or trisomies 18 and 13. The test is "positive" if the risk of Down syndrome is more than 1 in 301 or the risk of trisomy 18 or 13 is more than 1 in 150. 

What does the test tell me? 

Most of the time, test results will reassure you that your baby is not at risk for Down syndrome or trisomy 19 or 13. In other cases, the test results may say that your baby has an increased risk for one of these disorders.

It is important to remember that if your screening result is abnormal, it only means the risk is greater and you may need more tests. 

How accurate are the results? 

This combined ultrasound and blood test finds about 95% of babies with Down syndrome and pregnancies with trisomies 18 and 13. The test may also find other problems in the chromosomes. 

Keep in mind that this is only a screening test. It does not give a definite diagnosis. A normal test result does not guarantee that there is no mental retardation, birth defects or other problems that cannot be detected by this test. 

Do I have to have this test done?

No, it is your personal choice whether or not to have the test.

What if the results are abnormal? 

If this test shows an increased risk for problems, you will meet again with our genetic counselor. She will explain the test results in detail and describe your options for further testing.

These options include amniocentesis (a small amount of amniotic fluid is removed) and CVS (a small amount of tissue from the placenta is collected). The fluid or tissue is then examined for chromosome defects.

How is this test different from an amniocentesis or CVS? 

Amniocentesis and CVS are diagnostic tests that tell whether or not the child has Down syndrome, trisomies 18 or 13, or many other chromosome abnormalities and birth defects. Amniocentesis will also test for neural tube defects. These tests have a small risk of complications. However, they are 99.99% accurate. 

The first trimester  screening can help you decide whether or not to have an amniocentesis or CVS. 

Does this test check for spine defects? 

No, you should have a 2nd trimester maternal serum alpha-fetoprotein (AFP) screening for neural tube defects. 

What is Down syndrome? 

An extra chromosome 21 causes Down syndrome. This is why it is called trisomy 21. Down syndrome causes mental and physical retardation ("slowness"). 

Many children with Down syndrome also have birth defects in the heart or intestines. How much mental retardation and physical disability the child has varies greatly from person to person. 

Down syndrome can occur in the baby of a woman at any age. However, it is more common when a woman is over 35. Overall, Down syndrome affects one in 800 infants. Most babies with Down syndrome are born to women under age 35, because they have more babies overall. 

What are trisomies 18 and 13? 

Trisomy 18 and trisomy 13 are caused by having an extra chromosome 18 or 13. These changes in the chromosomes cause severe mental retardation and multiple birth defects. Most of these infants do not survive the newborn period. Very few survive longer than one year. As with Down syndrome, the change of having a fetus with trisomy 18 or 13 increases with the age of the mother.

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