Potential miscarriage

Has anyone had similar and then has gone on to have a viable pregnancy? Today I have had some light bleeding when I wipe. I called the early maternity unit and they said to come down for a scan. According to the NHS dating I am 8 weeks pregnant but that’s based on my last period. Based on my conception dates (I know which dates we had sex) I am 6 weeks. They said I’m measuring 6 weeks and baby is 3mm but they couldn’t see a heartbeat. When they say you’re measuring 6 weeks do they take in consideration that you’re not actually 8 weeks based on your period? I have another scan booked on 10/08 which feels so far away
Has anyone had anything similar and gone back and had a heartbeat?