There are a variety of approaches to screening. Some tests are carried out in the first trimester and some in the second. They may include blood tests, an ultrasound scan, or, ideally, a combination of the two.
Different tests have different detection rates. The detection rate tells you how good the test is at identifying babies who are truly affected as being high risk.
First trimester screening
First-trimester screening includes a blood test that measures the levels of two proteins in your blood and a special ultrasound scan called a nuchal translucency (NT) test.
The first-trimester blood test measures two proteins that are produced by your placenta: free Beta-hCG and PAPP-A. A woman who is carrying a baby with Down syndrome is more likely to have abnormal levels of these two proteins in her blood. The blood test can be done from 9 weeks through the end of 13 weeks.
The NT test measures the clear (translucent) space in the tissue at the back of your baby's neck. Babies with chromosomal abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger.
The NT screening can only be done between 11 weeks of pregnancy through the end of 13 weeks. (Because this test requires special training and equipment, it may not be available in many smaller institutions.)
Together, these tests are known as the first-trimester combined screening. They will identify as high risk about 79 to 90 percent of babies with Down syndrome (depending on which study you look at). This means that up to 21 percent of babies with Down syndrome will not be identified by this test.
The advantage of first-trimester screening is that you learn about your baby's risk for chromosomal problems relatively early in the pregnancy. If the risk is low, the results will offer you some reassurance.
If the risk is high, you may have the opportunity to consider CVS, which will allow you to find out whether your baby has a problem while you're still in your first trimester. (Keep in mind that CVS is usually done between 11 weeks and 13 weeks, 6 days, so to keep all of your options open, it's best to complete your first-trimester screening sooner rather than later.)
Second trimester screening
Second-trimester screening involves a blood test, commonly known as the multiple marker or the quadruple screen. This test is usually done between 15 and 18 weeks of pregnancy.
The test measures the levels of four substances in your blood: AFP, hCG, uE3, and inhibin A. Having abnormal levels of these substances in your blood is associated with an increased likelihood of carrying a baby with Down syndrome. The test detects about 80 percent of babies with Down syndrome.
Second-trimester screening is generally offered in conjunction with first-trimester screening as part of what's known as integrated or sequential screening (see below).
Integrated and sequential screening
Integrated and sequential screening involve both first- and second-trimester screening tests.
Integrated screening looks at the results of the first-trimester blood test, the NT measurement, and the second-trimester quadruple screen. It gives you a single assessment of your risk after all of these screening tests are completed.
Integrated screening detects 90 to 96 percent of Down syndrome cases. If the NT scan is not available, you may have serum integrated screening, which looks at the results of the blood tests from the first and second trimesters. This approach detects about 85 to 88 percent of Down syndrome cases.
Integrated screening is the most accurate screening method currently available. Unfortunately, you don't get any results until the second trimester.
Your caregiver may offer you an alternate strategy, known as sequential screening. With this approach, you get a first-trimester risk assessment after the NT screening and the first blood test. If the result indicates that your baby is at increased risk of having a chromosomal abnormality, you'll be offered diagnostic testing at that time, with the opportunity to consider CVS.
If the first-trimester test result is negative (meaning the likelihood of a problem is low), you'll be offered second-trimester blood screening and then get an adjusted risk assessment that incorporates all of the results from the first and second trimesters. This method is about 90 percent accurate in detecting Down syndrome.
Screening tests can identify babies at higher risk for a few problems in addition to Down syndrome. First-trimester combined screening can detect trisomy 18, trisomy 13, and a chromosomal disorder called Turner's syndrome. An abnormal NT scan result can also indicate certain birth defects, such as major congenital heart problems and abdominal wall defects.
Second-trimester screening can detect trisomy 18, and the AFP portion of the blood test screens for neural tube defects, such as spina bifida and anencephaly. (These are birth defects of the spinal cord and brain.) You'll be offered screening for neural tube defects (either the AFP blood test or ultrasound or both) during your second trimester if you've chosen not to have the full second-trimester screening or amniocentesis. Neither first-trimester screening nor CVS can detect these problems.