Quoting Amelia Margaret:" Testing for CF is normal but I don't know why they would want her back in a week. Perhaps the tech saw something that led her to believe there's more than one in there."
Quoting susanesque:" I don't know that they could see anything that would indicate CF on an early ultrasound. My friend's ... [snip!] ... would think it was just a precaution, like the other tests. They could for sure see twins by then, though. Good luck to her!"