How accurate are 12 week dating scans
Why are you concerned that your progress is different than what the scans show or the doctors think? To more properly address your question, it would be helpful to know why you are concerned?
They are charting average growth of a fetus, but cannot tell you with 100% when you ovulated or conceived.
Dating scans are the most accurate way of predicting your due date.
The whole point of the scan is to work out how many weeks pregnant you are.
It is not to provide the first photo for your baby album, or to find out your baby's sex.
DS1 my scan dates exactly matched the dates I had for conception, he arrived 8 days 'late' DS2 I had worked out the 18th of October, scan date changed that to the 14th, he arrived on the 13th.
DS3 once again scan dates exactly matched the dates I had worked out, he arrived 3 days 'late' DS4 I worked out the 18th of October (today) scan changed my due date to 17th. I think the scans are relatively accurate to ovulation (I think mine might be a couple days out but I can't be sure I didn't track) but I think at the end of the day it has little relevance as to when baby actually makes an appearance, there are 5 weeks a baby can be safely born in and I think that is more circumstantial than anything else.
the baby and its heart beat can be detected relatively easily with trans-abdominal and trans-vaginal examination.
This is presuming that the pregnancy is actually at this stage of development.