Early scan

Hello! Just looking for advice and any experiences really.

I had an early private scan today thinking I was 6w and 5d. I got pregnant through the 3rd round of leterzole and I ovulated late each time. I went off this pregnancy off the last period and thought I was at the 6w mark. Anyway when I went for my scan today they dated me at 4w and 4d and could see the baby and the sac and everything as it should but couldn’t see the heartbeat. I’ve got a rescan booked for two weeks time. Is this something I should worry about?