Empty Sac 6w

Mrs. B
I went to the ER yesterday due to some spotting and cramping. They gave me an u/s and the tech said she didn't see anything or she couldn't tell me. Before I was discharged the doc let me know that my hcg had went up and that he seen a 6w sac. Based off of my lmp I should've been almost 7w so there should've been a baby. Has anyone experienced this and then baby showed up a couple weeks later? Prayers would be greatly appreciated.