Not eligible for NIPT blood test because of 2nd empty sac.

My gynae said I’m not eligible for any Down syndrome bloodwork test because they detected a 2nd empty sac (no heartbeat) during my week 5 scan. The 2nd empty sac might mess with the result and a false positive is highly possible. They are only offering structural ultrasound (done around week 12/13) to check for signs of Down syndrome from the development of fetus. According to the hospital this is 80% accurate. Any mommies facing/ faced the same experience? Are there any other alternatives available with higher accuracy? #VanishingTwinSyndrome #nipttest