Confused! U/S said I wasn’t as far along

Maddie • Trying for baby #2! 👨‍👩‍👧

Went in for my first 12 week scan today I haven’t had a period since 5th September when it started, so today I thought I was 12w3d!! But the technician said I was measuring at 8w4d! Has this happened to anyone else?? I’m so confused! How could I have been 4 weeks off?!?!