How the due date calculator works
How the estimate is built
Naegele's rule: 280 days from the first day of the last menstrual period — because ovulation (and conception) happens roughly two weeks in, the fetus is actually ~266 days old at delivery. Longer cycles ovulate later, so the calculator adds the extra days; a 32-day cycle shifts the date by 4. This is exactly how clinics derive the preliminary date before any scan.
Why the date will probably move
First-trimester ultrasound (especially 8–13 weeks) measures the embryo directly and is accurate within ±5–7 days — better than any recall-based dating. Providers re-date when measurements disagree with LMP dating by more than ~7 days, and the ultrasound date then becomes official. A moving due date is not an error; it is the system working.
The window, not the day
A due date is a distribution's center: term is defined as 37–42 weeks, most births land within a week either side, and exactly-on-time is a ~5% event. Planning backward from the window (childcare, work, family travel) beats planning backward from the day — and the calculator shows the full-term window explicitly.