Understanding Your Triple Screen Test Results: What to Expect

Question
I am a 35 yr old mother of one six yr old and I am currently 4 months pregnant(lmp 12/21/03).
About 2 weeks ago I took the triple screen test. I was called by the hospitals genetic counsler to adivse me that the test result was positive! I am very worried... I nor the father of my child has any known history of birth defects. How accurate are the results of this test? I now feel I should do the amnio (even though I really did not want to!) HELP... Any information you can give me would be great.
Thanks

Answer
The triple screen measures the relative risk of neural tube defects (spina bifida, anencephaly, defects in the brain and spinal cord) and the risk of Down's syndrome. If the test shows a high value it may indicate increased risk of a neural tube defect, and if it has a low value it may indicate increased risk of Down's syndrome. There are many false positives and false negatives. At the age of 30, the risk for Down's syndrome is 1 in 2700 pregnancies. At the age of 35, the risk rises to 1 in 350 pregnancies. At  the age of 40 the risk rises to 1 in 100. Now, we usually recommend amniocentesis to any woman over the age of 35 (or who will be 35 when she delivers) because the risk of the procedure of amniocentesis results in miscarriage in 1 out  of 400 pregnancies, thus, when the risk of the disease is higher than the risk of the procedure, we recommend the procedure.  If they found that the risk of a NTD or Downs is higher than your risk based on age alone, then I, too, would recommend amniocentesis. Amniocentesis is the only 100% accurate method of detection. I hope I have put this into perspective. Good Luck to you.