But im not sure how it works tbh, always thought lmp was a more reliable date up until 12 week scan. Can anyone shed some light on this for. “My due date got put back by 2 weeks at the 12 week scan. Original date given was 18 December but at the scan it was moved to 31 Dec. It just doesn't quite add . Some advice and support would be really appreciated on this. I went for a scan at 7 weeks and 6 days. The ultrasounds measured the baby at 6.