My first thought is that 1:170 or 1:15 are both still really good odds for your baby.
There are also a few other things that pop up in my mind...
What "AFP" did you have? Even the quad screen is notorious for false positives. If you actually just had a AFP I wouldn't put much stock in it.
I'm also wondering why they did an AFP at all? The first trimester screening is so much more accurate than the AFP/triple screen/quad screen. While it can't screen for all the neural-tube defects like an AFP can, the 18-19-20 week u/s will pick up 99% of open neural-tube defects, anyway.
Finally, 1:170 is the DS risk for a 38-year old, so if you are younger than that your first trimester screening should have come back as a "screen positive" (even though 1:170 is a 99.4% chance that everything is fine). Did your doctor discuss those results with you at that time? To me the 1:170 on the 1st trimester screening is what possibly should have raised the question of an amnio, not the 1:15 from the AFP. Just seems weird to me that they didn't say anything about the results of your 1st trimester screen and also had you do an AFP.
Bottom line, though, chances are really, really, good that everything is fine. Hopefully your next u/s will show no markers and you can relax a bit until your amnio results come in.
My 18-wk u/s showed a bright spot in the baby's heart, which is a soft marker for DS. Since my DS results from the 1st trimester screening were good the doctor said he's probably OK and an amnio was "optional" (like, isn't it always!?), but I am going back for another ultrasound in June to look for more signs of DS that don't show up until 3rd trimester. If there are any, I'll have an amnio at that point to be sure. What I tell myself (although I have no medical confirmation if this is true or not) is that if everything else looks OK with the baby and he has made it this long even if he has DS he should be in pretty good shape and will be able to live a long and pretty healthy life.