Unsure of dates and size

[RNM1]

My last period was 12th December normally having a 29/30 day cycle. Tested positive strong positive on 13th Jan, one day after my period was due. Went for a scan yesterday assuming from my LMP I was 6 weeks however the scan only shown a gestational scan measuring 0.5mm suggesting 4 weeks, they didn’t seem concerned said this can happen with natural conception as we don’t know when ovulation happened. I retested today with clear blue digital which was 9 days since I last tested and it still reads 2-3 weeks, I thought it might of moved to 3+ weeks. I’m going for another scan in 2 weeks but driving myself insane with the uncertainty! Anyone else been in a similar position? Tia

I'm guessing you mean the sac measured 5mm? This is more in line with 5 weeks I believe?

If so, they said you were measuring approx 4 weeks on the 22nd, but you got a positive test on the 13th? This would mean you got a bfp at around 5dpo (if exactly 4 weeks). That's not possible. But if they actually meant you are "about 4 weeks but nearer 5" then it is possible.

I'd like to add that if this was a transvaginal NHS scan I'd give the measurement more weight than if it was abdominal or a private scan.