Hi philo, unless you were tracking your ovulation yes, your first scan can be miles off from your LMP date.
LMP is just a theoretical date that is used until more info is available by scan. I say theoretical because it is assuming an average woman's ovulation happens on day 14. As I am sure you are aware, we are far from average with our cycles so quite often it is wrong.
The best thing is to have another scan a week or two after your first and then see whether the measurement matches the first scan's. If it does, then your baby is growing perfectly on target and you adopt the scan's dating as the official age of your baby.
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