First Trimester Screening | Obstetrics | Dartmouth-Hitchcock
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First Trimester Screening

First Trimester Screening is an early way to estimate the chance that a developing baby has Down syndrome or trisomy 18.

The test uses an ultrasound measurement of the skin fold on the baby's neck (nuchal translucency) and combines this with the results of a test of your blood. Because this test is done between 11 and 13 weeks of pregnancy (during the first third of pregnancy), it is called First Trimester Screening.

What the test can tell you

This test can tell you whether you are at a higher risk for having a baby with either Down syndrome or trisomy 18. In general, about one in 20 women will have a "screen positive" test. If your result is positive, it does not mean for sure that your baby has one of these problems. Your provider may talk with you about other testing or refer you to a genetic counselor to discuss this in more detail.

First Trimester Screening cannot diagnose or rule out any specific condition; it is only used to estimate the risk of Down syndrome and trisomy 18.

Age and increased risk

Women of any age can deliver a baby with a chromosome condition such as Down syndrome; however, women who are 35 or older at delivery have a higher chance of having a baby with a chromosome concern. The majority of these conditions can be diagnosed by either chorionic villus sampling (CVS) or amniocentesis.

Why you might consider this test

One advantage to having First Trimester Screening is that you will have information about your risk for Down syndrome and trisomy 18 earlier in the pregnancy than you would with the standard Maternal Serum Screening. While both First Trimester and Maternal Serum Screening can test for Down syndrome and trisomy 18, Maternal Serum Screening can also test for neural tube defects.

The choice to have a screening test is personal. Your provider or a genetic counselor can help you decide.

If your result is negative (normal)

  • Your results will be sent directly to your provider and he or she will follow up with you.
  • This result means that the chance that your baby has Down syndrome or trisomy 18 is low enough that more testing is not suggested, but it is never possible to be certain that your baby will be born without birth defects. Even if your First Trimester Screening is normal, you can still consider other testing in the second trimester.
  • Please keep in mind that a normal screening does not guarantee a normal baby, nor does an abnormal screening result definitely mean a baby with a birth defect.

If your result is positive (abnormal)

  • A Dartmouth-Hitchcock counselor will call you within one week.
  • In some cases, chorionic villus sampling will be suggested. CVS takes a tiny tissue sample from the placenta. Because placental tissue has the same cells as the baby, testing the placental cells can tell whether the developing baby has Down syndrome or trisomy 18.
  • In other cases, you might choose to have an amniocentesis.

About these conditions

First Trimester Screening will help find about 84 percent, or 5 out of every 6, babies with Down syndrome, and about 80 percent, or 4 out of every 5, babies with trisomy 18.

  • Down syndrome, also called trisomy 21, is caused by an extra chromosome in the developing baby. It causes mental retardation and serious heart problems. One baby out of every 600 is born with Down syndrome. Although having a baby with Down syndrome occurs more often to women who are older, it can happen at any age.
  • Trisomy 18 is caused by an extra chromosome in the developing baby. Few babies with trisomy 18 survive to birth. One baby in every 6,000 is born with trisomy 18. Trisomy 18 causes mental retardation, heart defects, very poor growth, and other problems.
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