Low lying Sac

Hi all I just had my dating ultrasound at 8 weeks and got a due date of January 19 however in being asked to come back for a follow up ultrasound in 10 days as I have a "low lying sac" Has anyone else had this? What was the outcome? My doctor reassured me that it's fine and very common and not something to be concerned about but stupid be went to doctor google and it really sounds like a sign of miscarriage, unless I'm misunderstanding something