Trisomy 18

I am 12 weeks along. Around 9 weeks they did the genetic testing they offer to you and today I got a call back to come into the doctors office. He said im high risk for trisomy 18 and my levels were at a 2.5. He said he wouldn’t worry until around 7 or 8 but it’s still concerning me. Has anyone else had natera testing done and had this happen when it wasn’t true? They said the ultrasound looked really good at my 8 week ultrasound and on my testing except the trisomy 18 :(