Measuring a week behind at early scan but sure of dates

[Aria2015]

So first, here are my dates:
LMP 30.01.19
Positive opk 12.2.19
Last had sex 14.2.19
BFP 23.02.19 (10 dpp)

I have a pretty standard 28 day cycle which is always regular. I had a scan a week ago when I should have been 6 weeks 2 days but i measured a week behind (5 weeks 3 days) and all that could be seen was an empty sac. I was told that based on my dates, it didn't look good and was most likely a miscarriage but to come back a week later for another scan.

Had the second scan today and the sac had grown a week to 6 weeks 3 days and there was a yolk sac visible but no heartbeat detected.

The doctor didn't really know what to say other than for me to come back next week to see it any further changes had happened.

I'm so confused. Firstly, because of my dates. The doctor can't explain how i’d measure a week behind and secondly, surely by 6 weeks and 3 days (what the doctor measured me) they should be seeing a heartbeat?

I have zero expectation that this pregnancy is viable and hate the waiting game to get it confirmed. Has anyone been in a similar position?

6 weeks is borderline for a heartbeat from what I've been told

Was the scan external or internal? I think internal are more accurate but some places seem to not do them from what I've read on here

There is every chance you ovulated later than you think. Conception can be up to three/five days past sex, that's how long sperm lives.

Try not to panic.

Thanks for the replies. It was an internal scan so should be pretty accurate. I know i’ll have to wait for confirmation and if i hadn't been doing opk sticks then i might be a bit more optimistic but i just can't get it to add up.

It's still pretty early and you may have ovulated late or it may have taken a bit longer for the embryo to implant. It's very promising that the embryo continued growing though and 6 weeks is still early for a HB, so I wouldn't give up hope!

Best of luck for next week