Understanding a Positive Down Syndrome Screening Test (1:186) & Next Steps

Question
Hi, I am 30 years old and 18 weeks pregnant with my third child. My doctor called yesterday to imform me that my triple screen test came back positive for downs (1:186). I am scheduled for an ultrasound today. My concern is the false positive rate of these tests. My pregnancy with my daughter a year ago was very nerve wracking due to this. I also tested positive for downs then and the u/s showed she had echogenic bowels. The doc at the time (I am on a military base overseas) did an amnio which the tech passed out in the middle of. So, it wasn't the best experience for me. Luckily, the tests all came back clear of downs and any other infections. But with the changing of docs (a new one every 45 days... no permanent at the time) I had many different approaches and suggestions. I was told she would be a low birth weight baby and she was measuring much smaller than normal (my first child was 9 lbs 3 oz). She was born healthy and weighed 9lbs 5 ozs. My questions are... what should I be concerned about this time around? What are the chances of me having another false positive? What are the rates of one woman having 2 false postives? Is there anything else that could cause the triple screen to show postive just for downs? And, is the rate in which I was given for my chances a very high risk of having a baby with downs?

Thank you so much in advance!
~Stacey  

Answer
With only an AFP, there is a high chance of false positive results when testing for Down's syndrome because the normal curve overlaps the abnormal curve. However, as we perform a triple screen  (and the new quadruple screen) there are many less false positive results. You have to realize that the triple screen is merely a screening test and not a definitive result. The only definitive result can be obtained via amniocentesis. A sonogram may help if it shows nuchal cord transluscency, but that, too is a "soft sign" and can be a false positive. Unfortunately, there is no way to resolve the situation without having another amniocentesis. The results may show a normal karyotype, but you will be assured 100%.