How to calculate due dates for pregnancy:

There are a wealth of due date calculators and pregnancy due date charts on the web to help you estimate your due date. These tools will help you to estimate your date, they will not give you a 100% accurate date. Your baby will come when it is ready, what these tools do is to give the normal/average date that birth would occur based on the date of conception.

How accurate are the pregnancy due date calculators:

Like everything else, garbage in = garbage out. 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.