How Due Dates Are Calculated
The standard clinical method, called Naegele's Rule, adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a typical 28-day cycle with ovulation occurring around day 14.
If your cycle is longer or shorter than 28 days, this calculator adjusts the estimate accordingly, since ovulation timing shifts proportionally.
Why Due Dates Aren't Exact
A due date is a statistical estimate, not a deadline. Only about 4-5% of babies are born exactly on their calculated due date, according to clinical research. A "normal" pregnancy can last anywhere from 38 to 42 weeks — think of your due date as the center of a 5-week likely window, not a precise prediction.
Frequently Asked Questions
The standard method (Naegele's Rule) adds 280 days to the first day of your last menstrual period, assuming a 28-day cycle. If you know your exact conception date, due date is calculated as conception date + 266 days instead.
Due dates are estimates. Only about 4-5% of babies are born exactly on their due date — most arrive within a 5-week window, between 38-42 weeks gestation. Early ultrasound dating is generally the most accurate method.
Gestational age counts from your last menstrual period and is what doctors typically use. Fetal/conceptional age counts from actual conception and is usually about 2 weeks less than gestational age.