How it is calculated
280 days are added to the first day of the last period. If the cycle is longer or shorter than 28 days, the difference is added too. From conception, 266 days are added. From a scan, the remaining days are counted toward the same 280 from a notional period.
EDD = menstruation date + 280 + (cycle − 28)
Where the numbers come from
- What should I count from?
- Date
- Cycle length — unit in the field: days
- Weeks at the ultrasound date — unit in the field: wk
- And more days — unit in the field: d
Example
Last menstrual period on 1 January 2026, cycle 28 days. The due date is 8 October 2026, which is plus 280 days.
Questions
Why is the date approximate?
Birth rarely falls on the calculated day itself. That date is the middle of a window, not a schedule.
Which menstruation date should I enter?
The first day, not the last.
Does the calculator replace a checkup?
No. A clinician sets the timing and the plan from an exam and an ultrasound.