Second Trimester

Screening Tests

Screening tests do not involve any risk to the fetus or to the mother. Screening tests give results in fractions or odds of having a birth defect, based on the mother's age. So, although you will not receive a "yes" or "no" answer, women under the age of 35 will find out if their risk is as high as that of a 35-year-old woman. For women over age 35, the tests help them find out if the risk for their age is increased or decreased.
 
Screening tests performed during the second trimester include:
 
  • Targeted ultrasound
  • Maternal serum marker screening (triple test)
  • Nuchal translucency screening test (NTS).
     
Targeted Ultrasound Test
The best time to have this test is between the 18th and 20th weeks of pregnancy. Most major problems with the way your baby might be formed can be seen at this time. Some physical defects, such as clubbed feet and heart defects, may not be seen. Your doctor also will be able to see if your baby has any neural tube defects, such as spina bifida. In most cases, if you want to find out the sex of your baby, you can ask your doctor during this test. This test is not the most accurate for finding out whether your baby has Down syndrome. Only 1 in 3 babies with Down syndrome may have an abnormal second trimester ultrasound.
 
Maternal Serum Marker Screening ("The Triple Test")
This test, which looks for birth defects such as Down syndrome, trisomy 18, or an open neural tube defect, is usually given between the 15th and 20th weeks of pregnancy. It involves giving a sample of your blood to look for three chemicals produced by the pregnancy: alpha-fetoprotein (AFP), which is made by the liver of the fetus; and two pregnancy hormones, estriol and human gonadotropin. Higher levels of AFP are linked with open neural tube defects.
 
In women age 35 and over, this test finds about 80 percent of fetuses with Down syndrome, trisomy 18, or an open neural tube defect. In this age group, there will be a false positive rate of 22 percent (this means that there is a positive result but the fetus doesn't actually have one of these health problems). In women under age 35, this test finds about 65 percent of fetuses with Down syndrome, and there is a false positive rate of about 5 percent.
 
Nuchal Translucency Screening (NTS)
This new type of screening is offered between the 11th and 14th weeks of pregnancy. It involves an ultrasound exam to look at the thickness at the back of the fetus' neck combined with a blood test by sticking your finger to look at two proteins produced by the pregnancy. This test finds 90 percent of cases of Down syndrome and 97 percent of cases of trisomy 18. The false positive rate is about 5 percent. This test is not yet widely used, and not all doctors have access to it. In some cases, health insurance might not cover it. In England and the United States, doctors who use the test feel that it is a good option for many women.
 
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