Hmm.. OPK on 30th means you probably ovulated on the 1st or 2nd. Lets say the 2nd. Your bfp would then have been at 13dpo, so fetal age now would be 6 weeks, making you 8 weeks gone.
However, if the OPK was a false positive from your body gearing up for ovulation then stalling, you could have ovulated as late as the 6th or 7th (or later, but that pushes the bounds of the bfp) making it a very early 8-9DPO bfp. This would give a fetal age of 5w+1, and 7+1 'total'. Which could tie in with just an egg sac being seen?
I've seen posts on here with empty sacs/no fetal pole being seen on scans at about 7 weeks with positive outcomes, so I'd say it's possible everything is ok. I can't tell you how probable it is, but we're Brookers, so I'm sure everything will be fine!