LMP - The due date may be estimated by adding 280 days ( 40 weeks) to the first day of the last menstrual period (LMP). This is the method used by "pregnancy wheels". The accuracy of the due date derived by this method depends on recall by the mother, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle. Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7] hence the 40 week gestational period.
Length of cycles - To use LMP to calculate your due date you would also need to consider the length of your cycle. For example, you may have a 25 day cycle, 28 day cycle, 32 day cycle etc. The calculator will take this into consideration if you enter it correctly.
Conception - Conception occurs approximately 12- 24 hours after ovulation. Ovulation however, can vary from woman to woman though on average it occurs around day 14 of a woman’s menstrual cycle. To determine the due date from conception you would take the day of ovulation and add 38 weeks(or 266 days). This can be difficult to determine unless you are tracking your ovulation.
IVF - To determine the due date for IVF pregnancies you will need to determine the egg transfer date and determine if it is a 3 or 5 day blastocyst. The conception date is then calculated from subtracting 3 or 5 days from the transfer date then adding 38 weeks (or 266 days) can give the estimated due date.
Ultrasound - Ultrasound uses the size of the embryo to determine the gestational age (the time elapsed since the first day of the last menstrual period). The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age. The Ultrasound measurement of the embryo in the first trimester (up to and including 13 weeks 6 days of gestation) is the most accurate method to establish or confirm gestational age and due date, 8 weeks being the most accurate. The conception date can be approximated by subtracting 2 weeks from the first day of the last period. This again is not exact as ovulation dates may vary. The due date can then be calculated by determining the current gestational age and adding the amount of weeks/ days necessary to equal 40 weeks.