Understanding Elevated Nuchal Translucency & Down Syndrome Risk: Seeking Guidance

Question
The second sonogram told the back of baby's neck is 0.637 cms thick, which can be a risk for down's syndrome.  There is no other symptoms of down's syndrome from the ultrasound.  They are asking me to go for amniocentesis test which can lead to 1/250 chance of miscarriage at this stage.  I had my blood test done, I think for AFP, which came out normal.  We are confused whether to go for it or not.  Please help.
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Followup To
Question -
I'm 29 years old, 21 weeks pregnant with first baby, last menstrual period was Feb 19, 2004.

I had my 20th week ultrasound and my doctor called me and told that my baby's neck is thick(I may not be using the correct word that my doctor told me, since I was in a shock to keep it in mind).  They want me to go for a detailed ultrasound and sending me for that in 2 days.  What problem can it pose to the baby's health and how serious it is?

I'm worried since any ultrasound appointment will take atleast 3 weeks and they got me one in 3 days!  I guess it must be something really serious to worry about.  Please help me.

Thanks.
Answer -
Try to get a more specific diagnosis from the doctor. It would help to see the sonogram report. I can't tell exactly what was meant by a "thick neck". Most likely, your doctor wants to follow up on an unusal finding on the sonogram and to see if there are any abnormalities in the baby's neck. Wait to see what is found at the sonogram you are having in two days and ask for a written sonogram report. Then, if you have any concerns, let me know what was found.

Answer
The risk of miscarriage with amniocentesis is 1:400, but remember that the nuchal transluscency that was seen on sonogram is just a "soft" sign of Down's Syndrome. As long as the AFP was within normal limits, and you are 29 years old (which gives you a risk for Down's syndrome of 1:2700) and there are no other signs on the sonogram of abnormalities, I believe you are pretty safe in doing nothing. On the other hand, if you want to be 100% sure, an amniocentesis is the only way to know for sure. In 25 years of practice, I have not had any patients who miscarried after an amniocentesis. If it were my wife, I would not worry about the neck findings.