When my boy's TTT came back, he was diagnosed with 1:200 of DS. His detailed scan returned fine. Gynae suggested amnio but we refused to go for it, praying and trusting that he would be fine. Well, he turned out healthy and normal! The twins had no issues at all, not even jaundice.
The TTT accuracy was about 70% but I do understand the degree of stress it will cause to parents-to-be.