The early pregnancy
(5th or 6th week) scans are the most reliable in terms of expected date of delivery. Scans done at a later date i.e. in the mid trimester are not very accurate and may have an error ranging from 2-3 weeks on either side of the mean. So, a difference of 2 weeks now is not really significant. Besides, it also depends on the weight of baby and other factors like amount of liquor, parity, any gestational diabetes
etc. It is best to follow up with same sonologist and get serial scans. If there seems to be disparity of more than 2 weeks, after checking for other factors, it is also wise to monitor the condition of baby inside uterus
by doing biophysical profile
at the time of scanning.