Like everything else, a pregnancy due date calculator is only as accurate as the information supplied to it. These calculators work off the last period you had before you became pregnant.

If you don't know this date, it will be more difficult to estimate your due date. The calculation is based on the fact that conception most likely took place about 1 week after the first day of your LMP (last menstrual period). The gestational period is then about 9 months from that date

How accurate are the calculations
These calculations are only estimates. About 80 percent of babies are born within ten days of the due date. That leaves a fairly large window for error - about 10 days either side of the estimated date (20 days) and that is for 80% of pregnancies - there is another 20% which are earlier or later again.

Your doctor will be able to give you updated due dates based on scans of your developing baby . The measurements taken of the baby during these scans can be used in calculations to re-evaluate the estimated due date.