Most are born within a week either side of the estimated due date.
Some women prefer to count their pregnancy from the day that they were likely to be ovulating during the month in which they became pregnant.
There's no way to know for sure exactly when you ovulated and conceived.
As a result, most people, including health professionals, will date your pregnancy based on your LMP (last menstrual period) instead.
This will give you an estimated date of conception, which will usually be about two weeks after the first day of your LMP.
So you will be two weeks further along in your pregnancy if it's calculated according to your LMP rather than your estimated date of conception.