How to calculate your estimated due date of delivery
The method most frequently used is the one that assumes the conception occurring 14 days after the start of the last menstrual period.
The approximate date may be obtained by taking the date when your last period began, adding 7 days, then counting back 3 months and add a whole year. For example, if the first day of your last period was September 9, 2011; add 7 days and you get September 16, 2011; then count back 3 months and you have June 16: the estimated due date is June 16, 2012
Even though this is a good method to obtain your due date, REMEMBER that these are just ESTIMATED dates. In some cases your doctor can apply more reliable methods, such as: knowing the exact date of ovulation, measuring the size of the uterus by a clinical exam, ultrasound (during the first half of pregnancy, ultrasound can estimate the age of fetus within 7 days).