Screening Tests for Down syndrome…

There are screening tests that exist for neural tube defects and chromosome disorders (including Down syndrome) in a foetus. Screening tests can indicate to women that they are at an increased risk of having a child with a birth-defect, but the results aren’t definite, diagnosis tests are required to be certain.

Second trimester testing:
There is a blood test that is usually taken 15 – 18 weeks after the last menstrual period. This test measures the levels of three or four substances in the mother’s blood. The first is alpha-fetoprotein, which is produced by the liver of the foetus. Some is released into the amniotic fluid and passes into the mother’s bloodstream where its concentration rises gradually until late in pregnancy. These maternal serum alpha-fetoprotein (MSAFP) levels are noted. The levels of pregnancy hormones estriol and human chorionic gonadotropin (hCG) are also measured, and sometimes the levels of inhibin-A are also measured.

A woman’s individual risk of having a child with a birth defect is calculated, based on the levels of these substances as well as the woman’s age, weight, race, and whether she has diabetes requiring insulin treatment. Low levels of MSAFP and estriol along with high levels of hCG suggest an increased risk of Down syndrome.

If there is an abnormal test result, the next step is usually an ultrasound examination, which uses sound waves to take a picture of the foetus. If the foetus is older than suspected or there are twins, this may be what is causing the abnormal test results. If the ultrasound doesn’t provide an explanation for the test results, further diagnostic testing (such as amniocentesis or chorionic villus sampling) is recommended.


First trimester testing:
There is testing that can be done earlier, within the 11th – 14th week of the pregnancy. An ultrasound imaging exam called a nuchal translucency test can be used to measure the width of the foetus’ neck, when abnormalities are present, more fluid than usual tends to collect in this tissue. Blood test’s on the mother’s blood are also undertaken, looking for unusual levels of pregnancy-associated plasma protein A and human chorionic gonadotropic (hCG).

When this two-step screening is done during the 11th week of pregnancy, researchers claim that it can identify 87% of babies with Down syndrome. This first trimester testing enables the follow up diagnostic testing to be done earlier in the pregnancy, so a mother will know earlier if the foetus has Down syndrome.