R..
I had several miscarriages and one of my babies ended up going for genetic testing. It was a Trisomy issue, not 18, but it was totally incompatible with life. Only 3 or 4 children ever born in the US with it, ever. Yes, I Googled. (BUT I found out after the fact not before).
Let's look at the good that can come out of this. Information is power in planning for the future. (You are going to be stressed out for a few more weeks just in the stress of waiting...I am guessing that they will do an amnio, if you want to do more testing that way...It also sounds like a lot of Trisomy 18 issues should be able to be seen on ultrasound). Finding out early gives you the information to plan for the future. Do research. learn about any medical issues, know questions to ask at appointments, etc etc.
Honestly, all of my friends that have done the quad scan who have has "something" show up with different Trisomies have all gone on to have health typical babies with no genetic issues. I have heard this over and over with my friends through many different pregnancies. Sorry I can't tell you first hand that I have had this happen and everything was okay. But I have held friend's hand through this issue of waiting and things have always turned out well for them.
Good luck!! I will keep you in my thoughts!