18 days out between lmp and ultrasound dates

[Gemba78]

At my early scan of where I thought I was 7 + 1 turned out to be too small to scan but looked about 5-6 weeks. 12 days later when based on lmp date I assumed I would be 8+6 but was 6+2 (gestation not fetal dates)
Did a positive test on 12th April which would have made me conceive on 7th April which is five days earlier than test! Is this possible?
Only my second period following a tfmr but regular as clockwork normally
How can dates be so far out?
Next scan still two weeks away as going on holiday. Anxious as hell as had mmc directly before tfmr.
Has any one done a walk in scan in London area? How accurate
Any similar stories with dates widely out (good or bad)
Thanks x

Hi, I had a similar situation. My LMP was Feb 13th but when I went for an early scan they dated me as 6+4 when the LMP should have made me 8+6! Spent weeks worried and frustrated but at our 12 weeks scan (going by previous scan date) my baby was actually dated as 14+1. Luckily baby was still within size to carry out tests and measurements that they needed. My mum used work as a midwife and she would always date by LMP but it seems the new system is to stick to dates given at scans!

Thanks for the reply
Unfortunately my concerns were right and I miscarried sometime in the last three weeks
Had a scan on Monday to confirm then a d and c on Friday
Now going to see a specialist as it’s the third time
Hope you go on to have a successful pregnancy throughout
Xxx

So sorry to hear @Gemba78 best of luck with everything in the future xx