ultrasound scan

Hi ladies, so the hospital thought i was having an ectopic as I had some bleeding but when they checked they couldn't find an ectopic. So on Friday 17th they scanned me again and a gestational sac showed up (they are confident that's what it is) so they asked me to go back Monday 20th there was still nothing else in the sac but it had got a bit bigger. I have no idea how far I am as it don't add up with my lmp so I don't know if I ovulated a lot later.. my hcg levels was only rising by 44% so I've got to go back on the 27th for my final scan, should a yolk sac be present then?? Anyone had this before?? If nothing is there I think they are looking to end the pregnancy. Thanks 😘