Oh no, I totally understand why you're confused, the timing really does matter for OSCAR! From what I've gathered, the blood test component is meant to be done together with the nuchal translucency ultrasound scan, and the whole thing is supposed to happen between Week 11 and Week 14, so 27 March might actually still fall within that window depending on your exact dates. A few of us who went through KKH for first trimester screening found that AMC does tend to bundle the counselling, blood test, and scan all into one appointment, so it might not be a mix-up after all, just a different workflow from what you were initially told. That said, I'd really encourage you to call back and specifically ask the coordinator to confirm your exact gestational age on 27 March and whether it still falls within the acceptable first trimester screening window, just so you have peace of mind in writing. Definitely worth double-checking with your own doctor too, since every case is a little different especially with the vanishing twin history!