How a Due Date Is Actually Estimated From Your Last Period
The standard method (commonly called Naegele's rule) estimates a due date as 280 days (40 weeks) from the first day of the last menstrual period, based on an average cycle length and average pregnancy duration — it's a widely used estimate, not a precise prediction, and only a small percentage of births actually occur exactly on the estimated date.
This is general educational information about how the standard estimate is calculated, not medical advice — always follow the specific guidance of your own healthcare provider for anything related to pregnancy care and dating.
The standard calculation
Adding 280 days (40 weeks) to the first day of the last menstrual period gives the standard estimated due date. This assumes a typical 28-day cycle with ovulation around day 14 — for someone with a notably longer or shorter cycle, the actual conception date (and therefore true gestational timeline) can differ from what this standard assumption implies.
Why it's an estimate, not a precise prediction
Full-term pregnancies naturally vary in length, and only a minority of births occur precisely on the calculated due date — most happen within a window of roughly two weeks before or after it. The estimate is a useful planning reference and a starting point for prenatal care scheduling, not a guaranteed date.
Why an ultrasound can adjust the estimate
Early ultrasound measurements can refine the due date estimate based on the fetus's actual measured size and development, which sometimes differs from what the last-menstrual-period calculation alone suggests — this is covered further in the guide on why an ultrasound can shift the estimated date.