Possible Down's Syndrome

Let's start with my first two pregnancies having been high risk. The first worse than the second. I had preterm labor, was in and out of the hospital, and rough delivery follow by a couple weeks in the nicu. The second was just morning sickness (a lot) and she measured small so we did lots of follow ups and I ate every chance I got in hopes that I would keep something down eventually.
This time around everything has been great for the most part. The first trimester I had bleeding and cramps but baby stuck it out with me. 
I'm now 21 weeks along and just had the anataomy scan done. I got a voicemail regarding the results and need to schedule a follow up. Scaries voicemail ever. 
They said my blood work came back fine however the nuchal translucency measured large. I need to see a specialist and have another ultrasound done to measure other markers for down's. 
Has anyone had this experience before? The earliest they could get me in isn't for another couple weeks. 
My son, my first born has Autism. He was originally diagnosed with epilepsy and I was told about all the things he'd never be able to do. Which he has done them all! I have a great support system and I've always made sure he gets the therapies and services he needs to continue to progress. It's not always easy, but he is a blessing and I'll make sure he has whatever he needs. 
This possible diagnosis though, I just don't know. It is what it is and this baby will be loved no matter what and I'll already be in a good place knowing what places to go and people to talk to. Having two children with special needs just seems so very overwhelming at this point. We aren't telling anyone until more tests are done and we know more. I just needed to get it all out.