Here's the thing- a huge percentage of those tests will lead to false "positive" results, meaning something that freaks you out but comes to nothing. I had a similar scare when they found choroid plexus cysts on my second baby's brain at the 18 week ultrasound. The presence of these cysts means about a 1% chance of trisomy 18. Which seems small, but when they say 1 in 100 it sounds terrifying. Trisomy 18 is extremely rare. I found out that the biggest telltale sign is that on ultrasound, a fetus with trisomy 18 will not open its fist. When I went in and saw his little hand open and shut, I knew he would be fine :) Honestly, I would wait for tests. Go in for that 18 week check, watch the hands. The best diagnostic tool is an amnio, if you must know, balance the 1% chance of causing a miscarriage and look into that option. Otherwise, don't worry unless they see something on the ultrasound like problems with the heart, kidneys, cleft palate, clubbed feet, and fisted hands. Trisomy 18 is really terrible ("not compatoble with life" as I was told) and usually diagnosed well before a delivery date.