Hi mummy, that's quite a gap and I can understand why you're concerned. It could be a dating issue from the scan itself or sometimes the embryo just develops at its own pace, but honestly it's worth flagging this to your fertility clinic or doctor when you next check in so they can advise properly based on your full history. They'll likely want to scan again in a week or two to see the growth trajectory, which is really the most reassuring thing you can do right now.
Hey yes me. I conceived via IUI and was supposed to be at 7 weeks based on counting of last period. I did a scan last week and detected heartbeat for the first time, baby measuring at 6 weeks. My doctor is not worried about it so as long as the next few scans are developing correctly it should be fine!