- Most calculators use the average cycle length of 28 days. If your cycle is longer, shorter or irregular, these calculators won't provide an accurate estimate of your due date. Some more-accurate calculators allow you to input the average length of your cycle.
- Because it's hard to know exactly when you ovulate during your cycle, nearly all calculators utilize the 14-day rule and assume that you ovulate two weeks before the end of your cycle. Because this requires a more specific calculation, very few online calculators use this method.
- The average pregnancy is 40 weeks, or 280 days. All online calculators use this number to determine your due date.
- Many calculators use the first day of your last menstrual period as the start of your pregnancy. Calculators that use the length of the "average" cycle add 280 days to the first day of your last period. However, this can introduce anywhere from a few days to several weeks of error in the estimated due date.
- Some calculators will estimate the date you conceived by subtracting 280 days from your estimated due date. Because your due date is an estimate based on average cycles, the estimated date of conception can be extremely inaccurate.
