How reliable is a blood test to determine the presence of Down's syndrome in my unborn baby? --Jo

The blood test you are referring to is either an alpha-fetoprotein (AFP) test or a "triple screen" (combination test for AFP, human chorionic gonadotropin, and estradiol). The triple screen improves accuracy and is the test most commonly done now in the United States. In addition to Down's Syndrome (trisomy 21), the triple screen looks for signs of neural tube defects (spina bifida) and trisomy 18. Like all screening tests, an abnormal test result does not necessarily mean there is anything wrong, nor does a normal test rule out all birth defects. If there is an abnormal result on a screening test, further testing must be done -- usually ultrasound and/or amniocentesis.

In the case of Down's Syndrome, the triple screen detects about 60 percent of cases, with a 5 percent false-positive rate (the test indicates there may be a problem, but everything is normal). In women over age 35, the triple screen picks up 80-85 percent of cases of Down's Syndrome. Knowing how far along the pregnancy is vital in interpreting screening results. Therefore, the first thing to do in case of an abnormal test result is to verify gestational age by an ultrasound if one has not been done already. Other things that influence test results and are taken into account are your age, your weight, your ethnic background, history of bleeding in early pregnancy, and presence of diabetes. The test itself must be done during a relatively narrow window between 16 and 20 weeks.

The AFP or triple screen is a test that should be OFFERED to all pregnant women. Your health care provider should discuss with you the test's ability to pick up a certain percentage of some problems and its limitations in terms of false positives as well as its inability to detect all birth defects. The decision of whether to have the screening tests done is a very personal one.